Using a 555 timer to drive a piezo buzzer

Hi,

I’m trying to hook up a piezo buzzer (the one from the arduino starter kit) with a 555 timer.
I’m using the following instructions : http://makezine.com/projects/laser-tripwire-alarm/

The issue is that I’m only hearing a single “click” coming from the piezo when the light beam is interrupted instead of an actual sound.

The piezo works (I’ve hooked it up to an Arduino to play a melody) and the 555 timer also works because I can build a simple flashing LED.

Any idea what might be wrong ? I’ve followed the schematic and here’s how it looks on the breadboard :

https://dl.dropboxusercontent.com/u/13246619/555_piezo.JPG

Where is the schematic? That link dosn’t show it.

It's in the makezine article (step 2)

http://i1.wp.com/makezineblog.files.wordpress.com/2013/11/laseralarm.jpg?resize=620%2C465

Well if that is the schematic then it is wrong. You have no capacitor in it.

It could be you have the wrong sort of piezo. There are two types, you need the type that makes a sound when you just apply power. That is not the type you have.

I also noticed that and thought at first that it was an issue with the schematic but he did build the thing on both a breadboard and PCB without the cap. I assumed it would also work and wanted to do some measurements on it....

The single "click" noise is what I hear when I simply hook up 5V to the piezo. In fact I have several buzzers lying around here (different sizes / shapes) and they all generate that clicking noise. The only way I was able to get sound out of them was when I hooked them up to an Arduino and used "tone" to generate a melody.

I assumed that the 555 timer in astable mode would do the same thing.

His circuit also doesn't follow the standard "555 astable mode" like this one : http://www.instructables.com/id/555-timer-flashing-led-circuit/

I assembled that one and it worked fine, blinking the led every second. I thought hooking up the piezo would also generate some kind of sound but it just made a clicking noise. I assumed the 555 timers output pin (3) would generate a waveform suitable for the buzzer to generate a sound. Is it because the frequency (on/off cycle = 1 second) is too low for a piezo to generate a sound, and is that the reason I'm hearing a clicking noise ?

Is it because the frequency (on/off cycle = 1 second) is too low for a piezo to generate a sound, and is that the reason I'm hearing a clicking noise ?

Yes.

It most certainly is the sort piezo you have.

It looks like he's using the 555 as a comparator with latch rather than as a timer. Note the switch used to reset it.

This would make sense since the buzzer would only sound when the beam is broken - it would need to be latched. Clever use of a 555.

You do need a self-oscillating piezo sounder with that circuit.

VikkiFord: You do need a self-oscillating piezo sounder with that circuit.

Quite correct or use a 556 (dual 555) and use the second timer in astable mode to drive the existing buzzer. I find that most of the self oscillating buzzers have a horrible harsh sound compared to the other type.

How would one make a circuit in astable mode so that it works with a laser pointer.

Requirements :

  • When the beam is interrupted it needs to trigger the piezo.
  • Alarm continues when the beam is complete again.
  • We need to press a button / toggle a switch / turn power on/off to reset the alarm (turn the piezo off)

I've setup the 555 timer in astable mode and used a light resistive sensor to act as part of a voltage divider on an NPN transistor that powers the 555 timer. Obviously, when the beam is interrupted the alarm sounds, but as soon as the beam is again pointed to the photo resistor the alarm turns itself off. (as the timer is no longer being powered).

However I want the alarm to continue when the beam is re-established.

I noticed the following video where someone did a similar thing (555 timer in astable mode using a light resistive sensor acting as a switch).

http://www.youtube.com/watch?v=8BJ8TMnotCM

How would one hook something like that up ?

Oh .... is the 555 on the right (in monostable) mode triggered to turn on the second 555 on the left (in astable) mode ?

  • the monostable one is used to control the duration of the alarm
  • the astable one is used to control the frequency of the piezo / leds

I initially thought he was demonstrating 2 independent 555 timers. One monostable timer to trigger the alarm and another astable timer to trigger the alarm. Now just realise they are probably working together.

They are working together, quite correct. The monostable enables the astable for a period of time and then all goes quiet again.

I assume this is where someone just wants a heads-up to go look rather than have an alarm wailing until it's manually reset.

And how would one do an alarm that is kept on until it is manually reset ?

I assume it should be do-able with a single 555 timer chip but I'm having trouble coming up with a schematic to wire it together.

My setup currently only (briefly) triggers the alarm for as long as the beam is interrupted. I would like it to stay on until I use a switch ....

An alternative might be the latch (http://makezine.com/projects/laser-tripwire-alarm/) combined with a 555 in astable mode as suggested by UnoDueTre but that would also require 2 555 chips.

ddewaele: An alternative might be the latch (http://makezine.com/projects/laser-tripwire-alarm/) combined with a 555 in astable mode as suggested by UnoDueTre but that would also require 2 555 chips.

Two 555 chips are not required. The 556 has 2 x 555 in it.

Use the first 555 as per the original schematic (i.e. latching) but instead of that 555 driving the buzzer, it enables the second 555 (in the 556) which is configured as an astable (oscillator). This second 555 will then oscillate and drive your buzzer for as long as the first 555 is latched.

I assume it should be do-able with a single 555 timer chip but I'm having trouble coming up with a schematic to wire it together.

If you only want to use one NE555 you can do it if you replace the latching part of the circuit with two NAND gates or two NOR gates wired as an RS flip flop. Alternatively most forms of flip flop will do. For example the 74xx74 (xx is the technology) can be made to trigger on the rising edge of a signal (into the clock). Wire the D input to +5V and when triggered this will be transferred onto the Q output where it can enable the noise producing NE555. The only way to get the Q output back to zero is to put a zero o the ~Clear input. Pull it up to 5V with a 1K resistor and put a zero on it with a push button to ground.

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.

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 ?

ddewaele:
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.

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