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Need help with circuit. Trigger beep once when voltage is detected

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Jul 17, 2011
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Hi all,

I have an extremely cheap photography flash/strobe light that I want to modify. When the light is powered up, the light builds up a charge to fire the flash, it can be triggered at any time during the charge period but the problem is I wouldn't get the full power I set the light to. There is a "charge ready" indicator that illuminates when the desired power level has been stored and the unit is ready to fire. This indicator remains on until the flash has been fired at which point it turns off and the charging begins.

What I wish to do is play a short tone/beep once the indicator lamp has lit up. Eliminating my need to stare at my flash unit until I see that it's ready for the next shot.

My idea was to use a 1381U to detect when there is voltage present and triggering "something" to play the audible sound once and reset itself when the voltage drops.
Another idea was do use a 34-164 undervoltage to handle the trigger portion.

This video will give you an idea of what I'm looking for. Example of how i need my lights to alert me that they're ready - YOUTUBE

Hopefully I've made sense above and any help would be appreciated as I have searched and searched for a pre-made design that would work for me.


Thanks in advance.
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You may use the actual neon indicator by introducing a serie resistor in the circuit to lower the voltage for the detector circuit. This may power up a simple buzzer...

The detector circuit and circuit triggering the buzzer once each time the threshold is hit is what I need help with.

That's the idea:


  • flafh unit.GIF
    flafh unit.GIF
    19.7 KB · Views: 83

Here is monostable piezo buzzer for beep.............


A simple circuit, using a piezo buzzer:


  • Audio alarm.JPG
    Audio alarm.JPG
    26.6 KB · Views: 74

ok I'm looking at your circuit and kinda get lost as to how it works. I'm not an EE, just a hobbyist with no formal training. I see that you're using a voltage detection ic which would sense when there is current and trigger the tone but what's the other transistor? Does it break the circuit once the tone hits or will the speaker continuously buzz while the 1381 detects voltage?

Same moment the lamp is starting to light up, we have some voltage on the series resistor, we use the 1381 to detect voltage and the transistor to power-up the buzzer. Normally need to test before on the photo flash, as we don’t have any data for the current flowing through the circuit. Later to use a variable resistor instead to re-adjust the resistor value in order to obtain the voltage needed by the 1381.

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