Alarm 'on' or 'off' indicator

[paul_northam]

I'm looking for a circuit that can use the 12v DC output of my caravan alarm sounder. I am deaf and so am unable to hear if the set / reset alarm beeps have been activated. So, what I'm looking for is a circuit that will indicate with a red LED that the alarm has been set, and a green LED to indicate it is off, these LEDs can be sited inside a window. I know that the alarm sets with one beep and resets with two beeps, and so would need to use these input signals to activate the circuit. I am able to tap into the sounder for the 12v signal. Any suggestions anyone?

[pebe]

Is the sequence always the same, ie. beep.......beepbeep......beep........beepbeep....?
Is the frequency tone of the set and reset signals the same? If they are the same, the circuit will need to differentiate between the two. So how long is the single beep and how long is the double beep?

[paul_northam]

Yes the sequence is the same, one beep to indicate the alarm as set, and two beeps to indicate it to reset, although I have not measured the frequency or duration of the beeps, but I am advised (by a hearing friend) that the tone is the same for each beep from the sounder, and one beep possibly takes 500ms and the two beeps together take approx 1s.

[pebe]

I can think of a way of lighting the LEDs, but I need to clarify a few points.
1. Can you say what time period separates the two beeps when the alarm is reset?
2. Is the alarm signal a tone alternating between 0V and 12V?
3. Is there a 12V supply that you can tap into to power the 'adapter'?
4. Is the LED to stay lit until the next change of set/reset?

[paul_northam]

Hi pebe, my friend says that from what he can hear of the beeps, there is a separation of something like 2 - 3 ms between them.
Yes, the sounder does alternate from 0v to 12v dc.
Yes I am able to tap into a separate 12v supply.
It would be preferable for my benefit to be able to continuously observe the state of the LED's thereby allowing me to discern whether the alarm is set or reset.

Many thanks for looking into this for me, and I can say I have met 2 other deaf caravanner's who have admitted with embarrassment that their neighbours came running over to them to tell them that their caravan alarm is sounding.

[pebe]

Here is a circuit that should do what you want.

Two NOR gates of a CD4001 IC are wired as a bistable flip-flop. Normally pins 2 and 6 are both low so the bistable ‘sits’ at the last commanded position. Taking pin 2 high will make pin3 low and 4 high. Similarly taking pin 6 high will do the reverse. If both pins are taken high, both outputs will be low. The final state of the outputs is determined by which of pins 2 and 6 is the last pin to go low.

The 4001 outputs are only good for driving up to about 5mA, so the LEDs need drivers. They can be switched with transistors, but I find a 555 (wired as a bistable) is a convenient way to switch one of two LEDs using only a small footprint on the board.

In operation, D1 rectifies the incoming signal tone and C2 holds up the voltage on A. Pin 3 immediately goes low. Because pin6 is low, the output pin4 goes high (and pin1 keeps the LH gate on). Pin 6 of the 555 goes high resetting it and turning on the red LED.

For a single ‘Set’ pulse, C2 will discharge at the end of the pulse and the red LED will stay lit.

The next pulse coming in will be a double ‘reset’ pulse. Initially, the action will be the same as before, but this time the pulses are long enough to charge up C3 via R3 and VR1. Pin6 will go high forcing pin 4 low. That sets the 555, its pin3 goes high, and the green LED lights up.

At the end of the double pulse, C2 and C3 will discharge, but as C2 is smaller, point A will fall to zero before point B. The 4001 will be left with pin3 high and 4 low, and the green LED will stay on.

R1 is there to prevent any spikes that may be on the alarm signal from damaging the gate. Ground the unused input pins 8, 9, 12 and 13 of the 4001, but leave the output pins 10 and 11 unconnected. Use a good quality low-leakage cap for C3 - preferably a bead-type tantalum.

To adjust VR1, first touch the input to 12V to turn on the red LED. Then connect the input to 12V and keep it there. The green LED should come after about 750ms (about halfway through the second reset pulse). Adjust VR1 until it does.

Happy building!

[paul_northam]

Thanks enormously for that circuit pebe, I did try using a 555 timer to do the the job but kept failing to get it to work satisfactorily. I can now see how your design will do the job and will make it up and see how it goes from there. Just two points though, I said in my last reply that the separation between the beeps was in the region of 2 - 3 ms, I should have in fact said 20 - 30 ms, nonetheless I'm sure the potentiometer (VR1) should be able to accommodate the difference