Using a 555 timer to drive a piezo buzzer

[UnoDueTre]

Another idea, why not use a tiny85 instead of messing about with discrete gates and timers?
Has just enough pins (using the internal clock) to cater for everything including a user selectable jumper which would cater for both types of buzzer, i.e. the self oscillating types and the non.

A LDR and a preset (to set the trigger threshold)  are connected to the analog comparator.
The whole thing could be powered from a 9V battery via a 5V regulator.

Schematic and flow chart attached.

[ddewaele]

I'll take a look at that ... Don't have the Tiny85 at hand but I might order a few ......

I ended up building the following for now (2 555 timers, no transistor to switch) :

https://www.circuitlab.com/circuit/n9hsny/latched-laser-trip-timer-555/



The idea is that when the alarm should be activated, the output pin of the first 555 timer will be low.
The output pin of the first 555 timer is connected to the GND pin of the second 555 timer.
When the output pin of the first 555 timer goes low, the second 555 timer is grounded and thus it activates the piezo.

My initial idea was to "turn on" the second 555 timer by letting the first 555 timer apply by voltage to it when the alarm needs to turn on. However, when the alarm needs to go off, the output pin of the first timer is pulled low, so I ended by using that pin  to control the second timer.

Is it a good idea  (or even valid)  to have the LOW pin 3 (from the first timer) to ground the second timer in order to activate it ?

[UnoDueTre]

Is it a good idea  (or even valid)  to have the LOW pin 3 (from the first timer) to ground the second timer in order to activate it ?

I'm more worried about that 8 ohm speaker connected directly to the 555 output, are you not using a buzzer?
You could feed the low output from the first 555 to a PNP which will switch the supply to the second 555 (including the timing resistors going to supply).
Or even hold the reset of the second 555 low (also with a transistor) whist the first 555 is not triggered.

[ddewaele]

It's a piezo ... didn't find a piezo / buzzer symbol
Thanks for the additional insights....