Door Opener Alarm Circuit Diagram

The door opener derives its power from a 9-V battery. A momentary-contact switch, 52, is provided in the event that manual opening and closing is required. Relay Kl is a 9-V type and relay K2 is a 117`Vac latching-type, which automatically latches with the first burst of current and opens on the second burst.

Door Opener Alarm Circuit Diagram

Door Opener Alarm Circuit Diagram


The gate lead of the LASCR is not used; a light source triggers the LASCR unit into conduction, causing current to flow in the coil of the relay. That, in turn, causes Kl`s contacts to close, thereby energizing K2 (closing its contacts), and operating the garage door motor.
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Emergency Siren Simulator Circuit Diagram

This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail.

Simple Emergency Siren Simulator Circuit Diagram


Emergency Siren Simulator Circuit Diagram


The heart of the circuit is the two transistor flasher with frequency modulation applied to the base of the first transistor. When the pushbutton is depressed, the frequency of oscillation climbs to a peak and when the button is released, the frequency descends due to the rising and falling voltage on the 22 uF capacitor. The rate of change is determined by the capacitor value and the 100k resistor from the pushbutton.  The oscillation eventually stops if the button is not depressed and the current consumption drops to a tiny level so no power switch is needed.

The 0.1 uF determines the pitch of the siren: A 0.047uF will give a higher pitch siren and a 0.001 uF will give an ultrasonic (at least for me, anyway) siren from 15 to 30 kHz which might have an interesting effect on the neighborhood dogs! The 33k resistor from the collector of the PNP to the base of the NPN widens the pulse to the speaker giving greater volume.

The flasher circuit drives a PNP transistor which powers the speaker. This transistor may be a small-signal transistor like the 2N4403 in most applications since it will not dissipate much power thanks to the rapid on-and-off switching. The 100 ohm and 100uF capacitor in series with the speaker limit the current to about 60 mA and they may be replaced with a short circuit for a louder siren as long as the transistor can take the increased current. The prototype drew about 120 mA when shorted which is fine for the 2N4403.

Transistor substitutions should be fine - try just about any small-signal transistors but avoid high frequency types so that you do not end up with unwanted RF oscillations.
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Automatic Car Alarm Circuit Diagram

Even the best car alarm is useless if you forget to set it upon leaving your car, whence this circuit. The relay has a make and a break contact: the  former is necessary to delay the switching in of the  alarm after you have got out of your car, and the  latter serves to switch on the car alarm proper. Immediately on re-entering your car, you must press the hidden switch, Si. This causes silicon-controlled rectifier Thi to conduct so that the relay is energized. At the same time, the green LED lights to indicate that the alarm is switched off.  

Circuit diagram :

Automatic Car Alarm Circuit Diagram

 Best Automatic Car Alarm Circuit Diagram

As soon as the ignition is switched off, T, is off, T2  is on, and the buzzer sounds. At the same time,  monostable IC1 is triggered, which causes T3 to  conduct and the red LED to light. The silicon- controlled rectifier is then off, and D4 is reverse  biased, but the relay remains energized via its make  contact for a short time, preset by Pi As soon as this  time has lapsed, the relay returns to its quiescent  state, and the alarm is set via the break contact. The  delay time can be set to a maximum of about 1 minute.
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Water Alarm Circuit Diagram

The LM1830 fluid detector IC from National Semiconduc tor is designed to be able to detect the presence of fluids using a probe. This chip requires a relatively high supply voltage and is not the most frugal power consumer. It is also quite specialised so unless you are buying in bulk the one-off price is not cheap. 

An alternative circuit show n her e uses a standard CMOS IC type 74HC14. It has the advantage of operating with a 3 V supply and consumes less than 1 µA when the alarm is not sounding, this makes it ideal for use with batteries. 

Water Alarm Schematic Circuit Diagram

Water Alarm Circuit Diagram




The 74HC14 has six inverters with hysteresis on their input switching thresholds. A capacitor (C1) and a feedback resistor (R1) is all that’s necessary to make an inverter into a square wave signal generator. 

In the water alarm circuit the feedback resistor consists of R1 and the water sensor in series. R1 prevents any possibility of short-circuit between the inverter’s input and out-put. Resistor R2 defines the inverter’s input signal level when the sensor is not in water. Any open-circuit (floating) input can cause the inverter to oscillate and draw more current.The remaining inverters in the package (IC1.B to IC1.F) drive the piezo buzzer to produce an alarm signal. Capacitor C2 ensures that no DC current flows when the circuit is in monitoring mode (with the alarm silent) this helps reduce the supply current. 

A micro-switch can also be substituted for the water sensor to make the circuit a more general purpose alarm generator.



Author: Roland Heimann - Copyright: Elektor
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