Automatic Headlight Brightness Switch Project

Driving the highway with your high-beam headlights can really increase your visibility, but can be a blinding hazard for other drivers. This simple circuit can be wired into your headlight switch to provide automatic switching between high and low beam headlights when there is oncoming traffic. It does this by sensing the lights of that traffic. In this way, you can drive safely with your high-beams on without blinding other drivers.

Automatic Headlight Brightness Switch Circuit Diagram

Parts

Part    Total Qty    Description
R1              1         5K 1/4W Resistor  
R2,R3,R4   3         5K Pot  
Q1              1         NPN Phototransistor  
Q2              1         2N3906 PNP Transistor  
K1              1         Low Current 12V SPST Relay  
K2              1         High Current 12V SPDT Relay  
S1               1         SPST Switch  
B1               1         Car Battery  
MISC          1         Case, wire, board, knobs for pots  

Notes

Q1 should me mounted in such a way so it points toward the front of the car with a clear line of site. Suitable places are on the dashboard, in the front grill, etc.
Adjust all the pots for proper response by testing on a deserted road.
S1 enables and disables the circuit.
B1 is, obviously, in the car already.
Before you try to connect this circuit, get a wiring diagram for your car. Some auto manufacturers do weird things with wiring.
Connection A goes to the high beam circuit, B goes to the headlight switch common and C connects to the low beam circuit.

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Switch Mode Power Supply Circuit Diagram

The SMPS described here is suit-able for high-wattage stereos and other similar equipment. The circuit employs two high-voltage power transistors (BU208D) which have built-in re-verse-connected di-odes across their collectors and emitters. It can supply about 250-watt out-put. The circuit uses a ferrite core transformer of 14mm width, 20mm height, and 42mm length of E-E cores. An air gap of 0.5 mm is required between E-E junction. Good insulation using plastic-insulating sheets (Mylar) is to be maintained between each layer of winding. 

Circuit diagram:

Simple Switch Mode Power Supply Circuit Diagram

 

The number of primary turns required is 90 with 26 SWG wire. The secondary winding employs 17 SWG wire (for 4A load current). Each turn of the secondary develops approximately 2 volts. The reader can decide about the output volt-age and the corresponding secondary turns, which would work out to be half the desired secondary voltage. The volt-age rating of capacitors C7 and C8 should be at least twice the secondary output of each secondary section. BY396 rectifier diodes shown on the secondary side can be used for a maximum load current of 3 amperes. 

Two feedback windings (L1 and L2) using two turns each of 19 SWG wire are wound on the same core. These windings are connected to transistors T1 and T2 with a phase difference of 180o,  as shown by the polarity dots in the figure. First wind the primary winding (90 turns using 26 SWG wire) on the former. Then wind the two feedback windings over the secondary (output). Ensure that each winding is separated by an insulation layer.  Two separate heat sinks are to be pro-vided for the two transistors (BU208D). 

The filter capacitor for mains should be of at least 47µF, 350V rating. It is better to use a 100µF, 350V capacitor. If the output is short-circuited by less than 8-ohm load, the SMPS would automatically turn off because of the absence of base current.  The hfe min (current amplification factor) of BU208D is 2.5. Thus, sufficien base current is required for fully satu rated operation, otherwise the transistors get over-heated. At times, due to use of very high value of capacitors C7 and C8 (say 2200mF or so) on the secondary side or due to low load, the oscillations may cease on the primary side. This can be rectified by increasing the value of capacitor C6 to 0.01mF. 

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USB Switch Circuit Diagram

Anyone experimenting or developing USB ported peripheral hardware soon be comes irritated by the need to disconnect and connect the plug  in order to reestablish communication with the PC. This process is necessary for example each time the peripheral equipment is reset or a new version of the firmware is installed. As well as tiresome it eventually leads to excessive contact wear in the USB connector. The answer is to build this electronic isolator which disconnects the peripheral device at the touch of a button. This is guaranteed to reduce any physical wear and tear and restore calm once again to the workplace. 

Circuit image :
 

USB Switch Schematic Circuit Image

The circuit uses a quad analogue switch type 74HC4066. Two of the switches in the package are used to isolate the data path. The remaining two are used in a classic bistable flip-flop configuration which is normally built using transistors. A power MOSFET switches the power supply current to the USB device.  Capacitor C2 ensures that the flip flop always  powers-up in a defined state when plugged  into the USB socket (‘B’ in the diagram). 

The  peripheral device connected to USB socket ‘A’  will therefore always be ‘not connected’ until  pushbutton S2 is pressed. This flips the bistable, turning on both analogue gates in the data lines and switching the MOSFET on. The  PC now recognises the USB device. Pressing  S1 disconnects the device.

Circuit diagram :

USB Switch Schematic Circuit Diagram

The circuit does not sequence the connections as a physical USB connector does; the power supply connection strips are slightly longer than the two inner data carrying strips to ensure the peripheral receives power before the data signals are connected. The electronic switch does not suffer from the same contact problems as the physical  connector so these measures are not required in the circuit. The  simple circuit can quite easily be constructed on a small  square of perforated strip-board. 

The design uses the 74HC(T)4066 type analogue switch, these have  better characteristics compared to the standard 4066 device. The USB switch is suitable for both low-speed (1.5 MBit/ s) and full-speed (12 MBit/s) USB ports applications but the proper ties of the analogue switches and perf-board construction  will not support hi-speed (480 MBit/s) USB operation. 

The IRFD9024 MOSFET can pass a current of  up to 500 mA to the peripheral device with-out any problem.

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Touch Switches Circuits Diagram

TOUCH SWITCH 1

This circuit detects the skin resistance of a finger to deliver a awfully tiny current to the super-alpha try of transistors to show the circuit ON. The output of the "super transistor" activates the BC 557 transistor. The voltage on the highest of the world is passed to the front of the circuit via the 4M7 to require the place of your finger and also the circuit remains ON.

 To turn the circuit OFF, a finger on the OFF pads can activate the primary transistor and this may rob the
"super transistor" of voltage and also the circuit can close up.

TOUCH SWITCH-2

This circuit detects the skin resistance of a finger to show the circuit ON for concerning one second. The output are often taken to a counting circuit. The circuit consumes no current when in quiescent mode:

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Solid-State Switch For Dc-Operated Gadgets

This solid state DC switch can be assembled using just three transistors and some passive components. It can be used to switch on one gadget while switching off the second gadget with momentary operation of switch. To reverse the operation, you just have to momentarily depress another switch. 

The circuit operates over 6V-15V DC supply voltage. It uses positive feedback from transistor T2 to transistor T1 to keep this transistor pair in latched state (on/ off), while the state of the third transistor stage is the complement of transistor T2’s conduction state. 

Initially when switch S3 is closed, both transistors T1 and T2 are off, as no forward bias is available to these, while the base of transistor T3 is effectively grounded via resistors R8 and R6 (shunted by the load of the first gadget). As a result, transistor T3 is forward biased and gadget 2 gets the supply. This is indicated by glowing of LED2. 

Circuit diagram :

Solid-State Switch For Dc-Operated Gadgets Circuit Diagram

When switch S1 is momentarily depressed, T1 gets the base drive and it grounds the base of transistor T2 via resistor R4. Hence transistor T2 (pnp) also conducts. The positive voltage available at the collector of transistor T2 is fed back to the base of transistor T1 via resistor R3. Hence a latch is formed and transistor T2 (as also transistor T1) continues to conduct, which activates gadget 1 and LED1 glows. 

Conduction of transistor T2 causes its collector to be pulled towards positive rail. Since the collector of T2 is connected to the base of pnp transistor T3, it causes transistor T3 to cut off, switching off the supply to gadget 2) as well as extinguishing LED2. This status is maintained until switch S2 is momentarily pressed. Depression of switch S2 effectively grounds the base of transistor T1, which cuts off and thus virtually opens the base-emitter circuit of transistor T2 and thus cutting it off. This is the same condition as was obtained initially. This condition can be reversed by momentarily pressing switch S1 as explained earlier. 

EFY lab note. During testing, it was noticed that for proper operation of the circuit, gadget 1 must draw a current of more than 100 mA (i.e. the resistance of gadget 1 must be less than 220 ohms) to sustain the latched ‘on’ state. But this stipulation is not applicable for gadget 2. A maximum current of 275 mA could be drawn by any gadget.

Author : Praveen Shanker - Copyright : EFY

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Doorbell Warning Switch Circuit Diagram

This is the Doorbell Warning Switch Circuit Diagram. This circuit will light a lamp at a remote location when the doorbell switch is pressed. his circuit should only be used with the solenoid type doorbells, the electronic type that play tunes will not work here.  It is quite easy to miss the sound of a doorbell if you are watching TV , this circuit gets round the problem by providing a visual indication. As an alternative, a LED could also be used. You could just parallel a lamp across the doorbell, but this would mean extra drain from the doorbell batteries or transformer.

Doorbell Warning Switch Circuit Diagram

A series resistor, R1 is wired in series with the doorbell and reduces current flow, thereby increasing battery life. The value of R1 is chosen so that about 0.6 to 0.7 volts is developed across it, when the doorbell switch is pressed. I used a combination of a 22 ohm resistor in parallel with a 50 ohm. The voltage drop across R1 is sufficient to switch on the transistor, the lamp in series with the collector will then illuminate. I also used an electromechanical counter in parallel with the lamp.

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12v Light / Dark Switch Circuit Diagram

This is the 12v Light / Dark Switch Circuit Diagram. Often, for certain low voltage lighting systems; you would like to turn off the lights during the bright light of the day.  Most commercial day/night switches are designed for AC lighting.  This hobby circuit below was designed for a 12v DC system.  But, it could be modified for other voltage as well.  It uses an inexpensive photo-transistor as the light detector.  An n-channel FET is used to switch power to the lights.  A transistor circuit is included to provide some hysteresis.  

This keeps the circuit from fluttering the light during the transition from day to night and night to day.  It is recommended that a plastic tube be placed over the transistor to prevent it from being illuminated by the lights it is controlling.  By selecting the appropriate power FET, the circuit could control over 100 watts worth of 12v lighting.   (July 22, 2008)

12v Light / Dark Switch Circuit Diagram

Source: DiscoverCircuits

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Touch Switch II Circuit Diagram

This circuit uses a 555 timer as the bases of the touch switch. You can learn more about 555 timers in the Learning section on my site. When the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the buzzer for a certain period of time. The time that the LED and the buzzer is on is based on the values of the capacitor and resistor connected to pin 6 & 7. The 10M resistor on pin 2 causes the the circuit to be very sensitive to the touch.

Touch Switch II Circuit Diagram

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4A High-Speed Low-Side Gate Driver Circuit Diagram

This is the simple 4A High-Speed Low-Side Gate Driver Circuit Diagram. The UCC27518 and UCC27519 single-channel, high-speed, low-side gate driver device is capable of effectively driving MOSFET and IGBT power switches. Using a design that inherently minimizes shoot-through current, UCC27518 and UCC27519 are capable of sourcing and sinking high, peak-current pulses into capacitive loads offering rail-to-rail drive capability and extremely small propagation delay typically 17 ns.

The UCC27518 and UCC27519 provide 4-A source, 4-A sink (symmetrical drive) peak-drive current capability at VDD = 12 V. The UCC27518 and UCC27519 are designed to operate over a wide VDD range of 4.5 V to 18 V and wide temperature range of -40°C to 140°C. Internal Under Voltage Lockout (UVLO) circuitry on VDD pin holds output low outside VDD operating range.

4A High-Speed Low-Side Gate Driver Circuit Diagram
 

Features

• Low-Cost, Gate-Driver Device Offering Superior Replacement of NPN and PNP Discrete Solutions
• Pin-to-Pin Compatible With TI’s TPS2828 and the TPS2829
• 4-A Peak Source and 4-A Peak Sink Symmetrical Drive
• Fast Propagation Delays (17-ns typical)
• Fast Rise and Fall Times (8-ns and 7-ns typical)
• 4.5-V to 18-V Single Supply Range
• Outputs Held Low During VDD UVLO (ensures glitch free operation at power-up and power-down)
• CMOS Input Logic Threshold (function of supply voltage with hysteresis)
• Hysteretic Logic Thresholds for High Noise Immunity
• EN Pin for Enable Function (allowed to be no connect)
• Output Held Low when Input Pins are Floating
• Input Pin Absolute Maximum Voltage Levels Not Restricted by VDD Pin Bias Supply Voltage
• Operating Temperature Range of -40°C to 140°C
• 5-Pin DBV Package (SOT-23)

Device Uses

• Switch-Mode Power Supplies
• DC-to-DC Converters
• Companion Gate Driver Devices for Digital Power Controllers
• Solar Power, Motor Control, UPS
• Gate Driver for Emerging Wide Band-Gap Power Devices (such as GaN)

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Sound Activated Switch II Circuit Diagram

With this sound activated switch, control by sound may be very useful, not just on a robot but also for a bit of home automation, for example a sound-activated light responding to a knock on the door or a hand clap. The light will be automatically switched off after a few seconds. An alternative use is burglar protection — if someone wants to open the door or break something the light will come on, suggesting that someone’s at home. The circuit can work from any 5–12 VDC regulated power supply provided a relay with the suitable coil voltage is used.

Sound activated switch circuit diagram

When you first connect the supply voltage to the sound activated switch circuit, the relay will be energised because of the effect of capacitor C2. Allow a few seconds for the relay to be switched off. You can increase or decrease the ‘on’ period by changing the value of C2. A higher value results in a longer ‘on’ period, and vice versa. Do not use a value greater than 47μF.

Biasing resistor R1 determines to a large extent the microphone sensitivity. An electret microphone usually has one internal FET inside which requires a bias voltage to operate. The optimum bias level for response to sound has to be found by trial and error. All relevant electrical safety precautions should be observed when connecting mains powered loads to the relay contacts.

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12V Touch Switch Exciter Circuit Diagram

This circuit is designed to generate a 20KHz pseudo sine wave signal that can power about 50 remote touch activated switch circuits.  It can support a cable length of about 2500 feet.  A typical remote switch circuit is also shown as well as a receiver circuit for those switches.

12V Touch Switch Exciter Circuit Diagram

Source: DiscoverCircuits

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A Headphone Monitoring Switch Schematic

In any recording situation, monitoring is critical to make sure you're getting what you want on tape. This is just as true in field recording, but in most cases, one's monitoring options are severely limited--stereo headphone is the only choice.

Headphone Monitoring Switch  :

 

 

Since I often use dual-mono mics, hearing a stereo feed of the two is not always convenient. I wanted the option to hear JUST the left mic in BOTH ears, or just the right mic in both ears, as well as a normal stereo signal. This is simple enough to do with a big rotary switch. When completed, you can create a little box that your headphones plug into, which in turn is plugged into the stereo phone output of your deck. Then, by turning the knob on the switch box, you can hear normal stereo, left-only mono, right-only mono, left+right mono and even left-right reversed stereo (or normal stereo again). 

Note the use of summing resistors in the left+right mono section. This was an attempt to prevent the two outputs from "fighting" each other if there were very different voltages in left and right outputs. I used 8 ohm resistors here, but a higher value might be better. Maybe ~20 ohms? Also, I initially decided to put normal stereo on both ends of the switch's travel so I'd always be able to find it without looking. However, I sometimes wish to have left-right reversed. If you'd like to try this, simply swap the leads on one of the "normal stereo" connections. 

One final caveat: The left only/right-only mono positions are -6dB down, since only one half of the deck's headphone amp is driving your phones when the switch is in those positions. 

Source by : streampowers

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Auto Power Off Circuit Diagram

We are surrounded by battery operated equipment of all kinds, and this array is growing still. Manufacturers and designers lean over backwards to make sure that their equipment draws a small current and can thus be operated by a battery. This has its flip side, too. because even if the equipment in question draws only a small current, when it is not switched off, the battery is flat after a few days or weeks. The circuit presented here can prevent this happening. It may be added to all kinds of equipment operating from a 9 V battery and switches this off automatically one minute after a preset time has elapsed. The peak switching current is 20 mA, which is more than enough for most applications.

 Circuit diagram:

The switch is formed by a p-n-p darlington, T1, which is actuated by push-button switch S1. The very high amplification of the darlington enables it to be kept on fairly long with the aid of a relatively small-value capacitor, C1 (= 100 µF). Resistor R3 limits the charging current of C1 to ensure a long life of S1. Resistors R1 and R2, in conjunction with C1, determine the switch-on time. When this time has elapsed, R1 ensures that T1 is switched off. Since the darlington can handle a UBE of –10 V, a polarity protection diode is not needed.

Author:H. Bonekamp
Copyright: Elektor ELectronics

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Automatic Load Sensing Power Switch Circuit Diagram

This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).

Automatic Load Sensing Power Switch Circuit Diagram

Parts

Part	Total Qty.	Description
C1, C3	2	10uF 35V Electrolytic Capacitor
C2	1	1uF 35V Electrolytic Capacitor
R1	1	0.1 Ohm 10W Resistor
R2	1	27K 1/2W Resistor
R3, R4	1	1K 1/4W Resistor
R5	1	470K 1/4W Resistor
R6	1	4.7K 1/2W Resistor
R7	1	10K 1/4W Resistor
D1, D2, D4	3	1N4004 Rectifier Diode
D3	1	1N4744 15V 1 Watt Zener Diode
U1	1	LM358N Dual Op Amp IC
Q1	1	2N3904 NPN Transistor
K1	1	Relay, 12VDC Coil, 120VAC 10A Contacts
S1	1	SPST Switch 120AVC, 10A
MISC	1	Board, Wire, Socket For U1, Case, Mains Plug, Socket

Notes

• This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.
• A maximum of 5A can be used as the master unless the wattage of R1 is increased         S1 provides a manual bypass switch.
• THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.

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Automatic Load Sensing Power Switch Circuit Diagram

This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).

Automatic Load Sensing Power Switch Circuit Diagram

 

 

Parts

Part	Total Qty.	Description
C1, C3	2	10uF 35V Electrolytic Capacitor
C2	1	1uF 35V Electrolytic Capacitor
R1	1	0.1 Ohm 10W Resistor
R2	1	27K 1/2W Resistor
R3, R4	1	1K 1/4W Resistor
R5	1	470K 1/4W Resistor
R6	1	4.7K 1/2W Resistor
R7	1	10K 1/4W Resistor
D1, D2, D4	3	1N4004 Rectifier Diode
D3	1	1N4744 15V 1 Watt Zener Diode
U1	1	LM358N Dual Op Amp IC
Q1	1	2N3904 NPN Transistor
K1	1	Relay, 12VDC Coil, 120VAC 10A Contacts
S1	1	SPST Switch 120AVC, 10A
MISC	1	Board, Wire, Socket For U1, Case, Mains Plug, Socket

Notes

• This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.
• A maximum of 5A can be used as the master unless the wattage of R1 is increased         S1 provides a manual bypass switch.
• THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.

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Energy-Saving Switch Schematic Diagram

Lights do not always need to be on at full power. Often it would be useful to be able to turn off the more powerful lights to achieve softer illumination, but this requires an installation with two separately-switch-able circuits, which is not always available.

 Energy-Saving Switch Circuit Image 





If the effort of chasing out channels and replastering for a complete new circuit is too much, then this circuit might help. Normal operation of the light switch gives gentle illumination (LA1). For more light, simply turn the switch off and then immediately (within 1 s) on again. The circuit returns to the gentle light set-ting when switched off for more than 3 s. There is no need to replace the light switch with a dual version: simply insert this circuit between switch and lamp.

Energy-Saving Switch Circuit Diagram


Parts List:
Resistors:
R1 = 100Ω
R2 = 680Ω
Capacitor:
C1 = 4700µF 25 V
Semiconductors:
D1,D2 = 1N4001
Miscellaneous:
K1,K2,K3 = 2-way PCB terminal
block, lead pitch 7.5 mm
F1 = fuse, 4AT (time lag) with PCB
mount holder
TR1 = mains transformer, 12V @ 1.5
VA, short-circuit proof, PCB mount
B1 = B80C1400, round case (80V
piv, 1.4A)
RE1 = power relay, 12V, 2 x c/o,
PCB mount
RE2 = miniature relay, 12V, 2 x c/o,
PCB moun

How does it work?
Almost immediately after switch-on, fast-acting miniature relay RE2 pulls in, since it is connected directly after the bridge rectifier. Its nor-mallyclosed contact then isolates RE1 from the supply, and thus current flows to LA1 via RE1’s normally-closed con-tact. RE1 does not have time to pull in as it is a power relay and thus relatively slow. Its response is also slowed down by the time constant of R1 and C1. If the current through the light switch is briefly interrupted, RE2 drops out immediately. There is enough energy stored in C1 to activate RE1, which then holds itself pulled in via a second, normally-open, contact. If current starts to flow again through the light switch within 1s, LA2 will light. To switch LA1 back on it is necessary to turn the light switch off for more than 3 s, so that C1 can discharge via R2 and RE1. The printed circuit board can be built into a well insulating plastic enclosure or be incorporated into a light fitting if there is sufficient space.
PCB-Layout

Caution:
the printed circuit board is connected directly to the mains-powered lighting circuit. Every precaution must be taken to prevent touching any component or tracks, which carry dangerous voltages. The circuit must be built into a well insulated ABS plastic enclosure.

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Relay Toggle Switch Circuit Diagram

Half of RL1 and RL2 manipulate the switching and the other is connected to an application. Relays are 200 ohms above ground and at one point are referenced to positive that turns them off.

Description:

RL1 (which is off) applies plus voltage from its armature and latches RL2 “on”. The application terminals are set to [A]. The condition changes when S1 is activated, voltage is applied to RL2 latching RL1 “on” releasing S1 turns RL2 “off”. RL2’s armature is then directed to R1. Terminals are set to [B].

When S1 is pressed again, the relays negative side are referenced to positive, RL1 turns “off” (there’s no current flow). RL2 turns “on” when S1 is released, terminals are set to [A]. There is slight lag between relays depending on how long S1 is held.

Relay Toggle Switch Circuit Diagram

Note: 

If different relays are used, adjustment of R1’s value may be required. For example, OEG relays (12vdc, 270 ohm coil) need R1 at 60 - 70 ohms. The prime motivation for this design was to avoid using toggle switches for my audio control panel. Another plus, it can be controlled from a remote transmitted pulse.Link

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Toggle Touch Switch Using Two Inverter Gates

We  can make a simple touch switch using only two inverter gates, two resistors, and two capacitors. The schematic diagram of the circuit is shown in the figure below. At power up, the output (of U1A) will be high, and the inverting output will be low because U1A gate will be triggered to ground level by C2. After triggered, the low level of U1A input is maintained by U1B output via R2.

If we touch the pad at this condition, where the output is high, then the U1A input will go high because we “short” the voltage of C1 to the input pin, and the low level previously caused by low level of U1B output voltage connected via R2 can’t be maintained because our skin resistance is much lower than 10M.

After U1A input goes high then U1A output will go low, and now U1B will go high to maintain high voltage level of U1A via R2, so we can release our finger without loosing the last state. Touching the pad again after we release our previous touching will toggle the output as the condition is reversed.

After we touch the pad, we have to release before 1 second (R2C2 time constant) elapsed. If we touch the pad longer than R2C2 time constant then  the output will oscillate (about 1 Hz).

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Automatic Range Switching Circuit Diagram

You can pick up a 3½-digit digital volt-meter module nowadays for a little as a couple quid. This is a simple and expensive way to fit out a piece of equipment with an instrument. Most modules are based on the well-known ICL7106 IC . They operate from an ordinary 9-V battery, and they only provide a fixed measuring range (200 mV or 2 V). The accessory circuit described here converts a DVM module into a voltmeter with 20-V and 200-V measuring ranges, with the added bonus of automatic range switching.

This requires a ground-referenced symmetrical supply voltage (±5 V) instead of a battery. An inexpensive TL431C is also used to generate an adjustable reference volt-age from the supply voltage. The circuit described here uses an LCD module with a fixed measuring range of 200 mV. It has three pins for driving the decimal point; two of them are used here.

Automatic Range Switching Schematic

This is how the circuit works: IC1 converts the voltage to be measured by the DVM module into a ground-referenced voltage. This part of the circuit is based on a design idea from Carsten Weber [1] that was pub-lished in the June 2005 issue of Elektor Electronics.

If the input voltage is less than 20 V, the voltage divider formed by R1 and R4 reduces it by a factor of 100. Transistor T2 is cut off, so R3 has no effect on the division ratio. The voltage at the junction of voltage divider R8/R13 is 200 mV because the open-collector output of comparator IC2A is in the high-impedance state. If the input voltage rises above 20 V, IC2A changes state and the voltage at the junction of voltage divider R8/R13 drops to less than 20 mV. In response to this, the out-put of comparator IC2B goes high and T2 conducts. R3 is now connected in parallel with R4.

This yields a division factor of 1000 (200-V range). Of course, the larger division factor also causes the input voltage of IC2A to drop. To prevent this comparator from changing back to its previous state (which would cause the circuit to act like a sort of oscillator), the value of R10 must be chosen such that the voltage at the junction of voltage divider R8/R13 is less than 20 mV, as previously mentioned. The calculated value (with R10 in parallel with R13) is approximately 9.6 mV. In practice, the value is around 18 mV due to the resistance of the output transistor of the comparator.

This means that the circuit will switch back to the lower voltage range when the input voltage drops below approximately 18 V. The amount of hysteresis can be set by adjusting the value of R10. However, the circuit will oscillate if the value is too high. Film capacitors C1, C3 and C4 sup-press noise and create a certain amount of inertia for range switching. This prevents frequent back-and-forth switching in the threshold region.

The other two comparators of IC2 sup-ply mutually complementary output levels that depend on the measuring range. The associated decimal points of the DVM module are driven via p-channel FETs.The circuit has two trimpots: P1 is used to correct for the offset voltage of the operational amplifier (IC1), while P2 is used to set the threshold level for range switching For this purpose, first adjust the trimpot to produce the maximum possible reference voltage (around 3.4 V). Next apply an input voltage that causes a display reading of 19.99 (which ideally means 19.99 V). Now turn P2 until the measuring range switches.

As a check, reduce the input voltage to force the measuring range to switch back, and then slowly increase the input voltage again. The ideal setting is reached when the measuring range switches before the DVM module displays an ‘overrange’ indication.

Author : Rainer Reusch - Copyright: Elektor

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Multiplexer Switch with 4066 Circuit Diagram

This is a switch circuit using a digital multiplexer 4066 Ic, their keys are ES8 ES1 to 4066 and each contains four switches. When the channel is selected first, and ES1 ES2 ES3 are closed and open, which prevents cross-over between the two channels is the grounding of the channel is not in use.

The bandwidth of the multiplex circuit is about 8 MHz and the consumption is 1 mA. When the multiplexer circuit is connected to a load impedance of 75 Ω, the circuit will result in some loss of signal due to internal resistance of the keys. This can be compensated for by connecting an amplifier output.

Multiplexer Switch with 4066 Circuit Diagram

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