Kitchen timer to open relay help please.

[ItsTheH]

Hi all, and thanks in advance for reading and for any assistance offered.

I am fairly new to electronics and have built a few circuits from schematics recently, and am now looking to try some things that I would use for my airsoft site (skirmish) in the UK.

The 1st one is as follows :-

I would like to use a common electronic kitchen timer with a piezo buzzer on it (taking the wires from the piezo), amplify the voltage and then turn the voltage from small pulses to a constant signal for enough time to switch a relay.

After quite a bit of reading, it seems I need to pass it through an operation amplifier and a monostable multivibrator before it hits the relay. I wouldn't mind a bit of trial and error but I live a long way from any electronics shops and purchasing the wrong parts will result in long delays and (I would imagine) frustartion.

The kitchen timer only gives off about 20 milivolts through the piezo.

I would like to use a 12 volt power source for this circuit.

If anyone can help me out, I will be forever in their debt!

Just to try and help people understand what I am trying to achieve, I would like to build some units that can be set for up to an 8 hour countdown, and when they hit zero they will blow an electrically operated pyrotechnic (9 to 12 volts)

Hope you can help,

Howard

 

[Hero999]

The timer will send a lot more than 20mV to the piezo, probably at least 1.5V, maybe more.

There's no way a multimeter will be able to accurately measure it, you need an oscilloscope to do that.

The voltage to the piezo will be AC. It needs rectifying so it can switch the relay on.

Do you want the relay to stay on and remain on, once the piezo has sounded, until you press a button to reset it?

If so, an SCR could be used to turn the relay on - it's pretty easy to do.

 

[ItsTheH]

The kitchen timer I am talking about is the small cheap battery operated type, but I don't have access to an oscilloscope.

I am (just about) adept enough to work out the scr (ahhh google you saved me again). If it does give off 1.5 volts, is this enough to switch a relay (I can't seem to find one that operates at that low a voltage)

I would not need the relay to stay open, as long as it was on for say 2 seconds at least then that would satisy my needs.

Thanks for the quick response.

 

[Hero999]

Do you mean open or close?

You could use a relay with normally closed contacts?

How are you going to power the relay?

If the timer's battery voltage is too low or the battery isn't large enough, you'll need to use a separate battery to power the relay.

t = 1.1RC

For 2s use 82k and 22µF for R and C respectively.

 

[ItsTheH]

ya, sorry, I did mean closed, not open and I intended to run the relay from a seperate power source (12 volts) as the timer works off of 3 volts.

Thanks for your helps, I will buy the components recomended and see if I can get it working. Thanks a lot for your help and I will post more work for you once I have this sorted.

 

[ItsTheH]

Before I purchase the parts, could someone just check that I have everything correct.

2 x diodes
1 x 10k resistor
1 x 82k resistor
1 x 100k resistor
1 x 555 timer
1 x 22uF capacitor (polarised)
2 x 10nF capacitors (non polarised)
1 x 12v relay
1 x NPN transistor (such as a 2N2903)

I think that's it.

 

[Hero999]

That's right, let us know how you get on.