This will be easy for someone out there... - RFEDA Forum

I have a problem as follows:A friend of mine has a sounder wired up (i think it's called a piezo sounder, you know, like the brass disc with a white disc in the middle of it) to let him know when someone drives up the path to his farm. Right near where the sounder is (outside) is a barn where he often works with machinery. He sometimes misses the sounder when the machinery is on so he wants a blue strobe (like off a police vehicle) that operates when the sounder goes off and stays on till he resets it.I am an electronics virgin but I have a degree in Physics and I am an electrician so i hope i can manage to sort something out. Any advice on how to do it?I was thinking that the strobe needs a constant voltage and the voltage to the sounder is oscillating between 1-12V i think, so it would be best to feed the strobe from it's own supply in the workshop and just trigger it with the voltage to the sounder. That's where i need help. I can do the circuit and reset button etc from a relay of sorts, but i don't know how to trigger the relay with the oscillating voltage. Any ideas? agwk
agwk

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1	htc2498	Sorry, forgot to mention, i also want the circuit timed so that it only stays on for 10 minutes or so. That way, if he is not in the workshop, the strobe doesn't stay on all day until he resets it.Thanks.	2008/9/3 20:56

2	rickli	Its late and had a few NYE bevvies...... but perhaps you could tap off the output of the buzzer to say a relay which triggers another cct that powers the strobe. When there is no output to the buzzer the relay closes and shuts off the strobe....Damien	2008/9/3 20:56

3	gao51755775	Some electrician you are. Use a one shot timer..	2008/9/3 20:56

4	hualeyan	Point Taken. LOLDamien	2008/9/3 20:57

5	booksir	A one shot timer????????? Never heard of it.And using the oscillating feed to the sounder to trigger a relay, will a relay hold in when the voltage isn't constant.HAPPY NEW YEAR by the way.	2008/9/3 20:57

6	ct731	Hi,QUOTE (stakhanov)And using the oscillating feed to the sounder to trigger a relay, will a relay hold in when the voltage isn't constant.You only use the piezo to trigger a timer - eg. like this, just with a longer time constant (C2/R4):	2008/9/3 20:57

7	lassa	That looks great Soeren, thanks.The only problem is knowing what all the components are and their values (and what VDD etc. means)... and how the time constant relates to the actual time. Any help with that?Thanks.	2008/9/3 20:57

8	netlgc	stakhanov, rather than taking the trigger for your strobe circuit off the piezo sounder, I'd recommend looking for a more suitable trigger signal at a point earlier in the vehicle detector circuit. While it is definitely feasible to trigger something off the piezo circuit, that circuit is designed to drive a specific, high impedance, capacitive load (the piezo device), whereas there is probably a low impedance trigger signal directly at the output of the vehicle detector circuit or at an intermediate point that turns on the piezo driver oscillator for a programmed time period. This would be the optimal point to pick off a trigger to turn on your strobe circuit. Finding that point, however, may require a circuit block diagram, a schematic, or a little sophistication in visually examining a circuit board to determine what is happening at various points.If you can find the point in the circuit at which a moderately low-impedance voltage change takes place when a vehicle is detected, you could use that signal to trigger a timing relay directly or via a transistor or MOSFET, depending upon the voltage and current available at the trigger source and the trigger requirements of your relay.If you examine the circuit board, you might be able to find the circuit block that generates the oscillating voltage to the piezo. Then look for the circuit immediately preceeding the oscillator that supplies the actuation to the oscillator. That is probably a good point to pick off a signal to your strobe circuit controller because it probably already has a one-shot timer on it so the piezo stays on long enough to get the attention of the listener while not staying on permanently. If that is the case, and if the time period is appropriate, just use that signal to close a simple relay that switches on the strobe unit.If there is no built-in time period or the built-in time period is inappropriate, use the trigger signal to close a one-shot relay, as suggested by PointGiven. You will need a timer function described as OFF-Delay or One-Shot, preferably with isolated trigger input. Most ordinary "time delay" relays are ON-Delay, which simply means they do not close for a programmed time after power is applied to the relay. This function is NOT ideal for your task and would require extra circuit tricks to use. With isolated trigger input terminals on an ON-Delay or One-Shot timer, you can simply provide an external contact closure or transistor switch to close the relay contacts and they stay closed (activating the strobe) for a period you program with switches, a pot, or a fixed resistor.Look at the OMRON timers in the MOUSER catalog (www.mouser.com), but check for the required function before buying. IDEC, Potter and Brumfield, and many others make comparable timers, and you can buy very versitile timer kits from Vellemann and other kit suppliers on the 'net. While you can definitely build a custom circuit to do the job, I think wiring up a time-delay relay is much more suited to an electrician's skills than fabricating a circuit board or even building a kit (unless you want the fun of doing it). The timing relay will do the job just as well as a custom-fabricated electronic circuit with a lot less effort and a much shorter learning curve.Note that you can also use the AC piezo drive signal to turn on a MOSFET to trigger the timer, but that will take just a little more sophistication to buffer the signal, so why not find the more suitable point with a DC shift within the circuitry?Good luck.awright	2008/9/3 20:57

9	ybyygu	Why not use relays to make a simple latch circuit?That way you can have a seperate power supply to the strobe.A degree in physics and you're an electronics virgin??What college did you go to!?	2008/9/3 20:57

10	z315	Hi,QUOTE (stakhanov @ Jan 3 2006, 01:39 PM)The only problem is knowing what all the components are and their values (and what VDD etc. means)... and how the time constant relates to the actual time. IC1 = 4011 or 4093D1 to D3 = 1N4148 or similarR1 = 100kR2 and R3, 10k should doC1, 10 to 100 nFC3, 100 nF or largerR5 = 4k7 or thereaboutQ1 and Q2, any small signal transistors or make Q2 a power darlington.Vdd is your positive supply (5 to 15V)C2 and R4:Time [seconds] = ln(2) * R * Cln(2) (the natural logarithm of 2) is around 0.693Eg. R4 = 1 MOhm and C2 = 1F0.693 * 1,000,000 Ohms * 0.000001 Farad = 0.693 second(With R4 = 1.5 MOhm, it will be very close to 1 second/F).	2008/9/3 20:57
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