Handy Dandy Little Circuits #17
            

Download # 17 in PDF


There are a lot of alarm systems on the market but you might be inclined to build your own. This little project can be put together using inexpensive parts readily available. The alarm transmitter module allows for a delayed trigger and auto turn-off as it is designed assuming that once the remote alarm has been triggered it will remain on until it is turned off manually.

Looking at the circuit switch S1 can be a open contact switch, a magnetic switch or other normally open switch of your own choosing for the application you have in mind. As soon as S1 is closed there is a time delay determined by R1-C1. This provides an allowable reset time for exit and re-entry to deactivate the alarm with the " Reset button " before it is triggered by the CD4001 gates (C-D). If there is no reset the gates (C-D) trigger Q1 and the tone generator (A-B) set -up as an oscillator ( the frequency of which is determined by R3/C3 ) . Q1 turns on the Opto-isolator (TIL112) which powers the Transmitter module via a 9V battery. It will remain on for the duration of the oscillation being transmitted.

After a delay determined by R2-C2 charging up to the positive DC rail gates (C-D) trigger will shut off the oscillator (A-B) and Q1 which in turn will turn off the supply to the transmitter via the opto-isolator reverting to the monitor mode where only a fraction of a miliamp is used from the battery in that mode.
You may choose your own time delay by changing the values of R/C as noted on the schematic.

You may decide to build several small alarm units after your success with the first one. One way of identifying each of these transmitters is assigning a different tone for each unit by changing the frequency . To do this R3 at points (A) as shown on the oscillator portion of the CD4001 (A-B) must be changed . While using a fixed capacitor C3 at point B an encoder could be substituted at A and B in series with C3 instead of the present fixed R3 . Such an encoder is shown made up with a 14 pin DIP for seven switches, actually, four would probably suffice using parallel switching for different chosen resistor values for each frequencies . On the other hand , if each transmitter would be assigned its own frequency with fixed R3/C3 this would simplify the construction and do just as well.
As for the transmitter I have used the Radio/TV FM transmitter in #3b and a small portable FM radio taking the signal from the headphones output for processing and it worked very well. The transmitter doesn't need an antenna for close range and it tunes the lower FM band around 88mHz with the variable capacitor. There are now many new types of miniature Tx/Rx modules that can be had from electronic suppliers that could be used and greatly simplify the project.

The FM "Mini- receiver" on this page could be used , R1 (PC type) controls the regeneration and after adjusting for best reception it should be locked in place with a dab of silicon as you may need to readjust it later. The 25pF variable capacitor tunes the selected frequency and R2 provides the load and must be adjusted for maximum output to an amplifier stage without distortion. Another option worth considering is the RF Detector described in Handy Dandy Little Circuit #15b.

On page two I describe the application of the LM567 as tone decoder.

See page #2 for more applications

See page #3 for PC layouts