Wireless Alarm
Circuit : Andy Collinson
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The two alarm circuits on this page are both silent alarms. They are low power, short range (30 to 40 feet) and make no noise where they are installed. They need no wires and can be used to monitor a garden shed or other remote building. In the home a small FM radio is used to listen for the signal. Should the alarm be triggered then a tone is heard on the radio.

The first alarm is powered from a 9 Volt battery. In standby it draws no current and when triggered draws around 40mA. The trimmer C8 is used to adjust transmitting frequency. As this is not a crystal controlled transmitter frequency will drift with battery voltage so can be powered from a 9Vdc power supply.

The circuit is triggered by S1. This would be a lever micro switch as shown left. A suitable part is available from CPC Farnell. The switch would be attached to the door and the lever bent so that when the door is opened, the switch closes, and when the door is closed the switch contacts are opened. The contacts are normally marked NC for normally closed and Common for the other terminal. Also note that this circuit is not latched, in other so only transmits when the door is open. As its designed to be used in a remote location and makes no noise. anyone forcing entry would not know this.

The amount of modulation can be adjusted by VR1, strong modulations may produce a harsh tone. The transmitter power is kept at a low level and the antenna should not be longer than 30cm or you can use a telescopic whip. It will be illegal in many countries to use an unlicensed transmitter so this is another reason for using low power.

Looking at the output signal, shown above, a modulated carrier is produced across L1. To see the frequency a fast fourier transform FFT can be performed and results shown in LTspice below:

The FFT shows the output spectrum and harmonics, the large centre spike is approximately 96Mhz (but variable by tuning the trimmer). There are also some harmonics but these are 30dB lower than the fundamental (96MHz). Do not be concerned about the harmonics because they will be very weak and the gain of the transistor falls off after 300MHz anyway.

The second circuit is shown below and very similar to the first circuit. The output tone can be adjusted by preset PR1 and the amount of modulation adjusted by PR2. The transmitter circuit is the familiar colpitts oscillator which has been described before.

Q1 can be any general purpose transistor but and RF transistor such as the MPSH11 wil give better performance at higher frequencies. MPSH11 or MPSH10 can be used. Stockists of MPSH10 are Bowood Electronics in the UK and Mouser.com worldwide.

Coil winding details can be found on other pages, e.g. the Mousetrap circuit and calculators for other values of the tank circuit may be found here.
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