Hello Everyone,
I'm brand new to your forum and learning about electronics by reading and building circuits. I have a beginners question regarding diodes. I have put together a circuit that I found here:

http://www.ecircuitslab.com/2011/06/555-timer-circuit-with-variable-onoff.html

I cannot get it to work although I have rechecked it multiple times. Using an LED there is voltage at all times at pin 3, which I can vary with either pot, but not get it to switch.

The circuit requires 1N914 diodes but I used 1N4003, which I had from another project. Is it possible that the diodes are at fault. I would appreciate your opinion.

Thanks - Geoff

 

Hello and welcome.

Using a comparitively slow rectifier diode for a fast switching application is possibly why it's not working. I don't know how fast the 1n4000 series switches though to be sure so that might be total rubbish.

You could also use 1n4148 if that's easier. I've certainly foun more of them in the old electronics I've stripped of parts.

 

With the values shown the frequency is about 65Hz so the leds will be lit dimly, if you want to slow it down to make an led flash
, change the capacitor to 10uF instead of 100nF

 

I'm assuming you are connecting your LED to the output of pin3 and routing it to ground with an appropriate series resistor. If not then you are blowing the LED with too much current. If you are using a resistor then there must be an error in your circuit.

The circuit works fine in Multisim (left 555 only) and the LED I put on the output flashes away merrily as you'd expect.

 

Add some bypass caps.
Add a 0.1 uf ceramic cap from V+ to ground, and add .01 uf capacitors from pin 5 to ground on at least the 1st (left) 555. Some 555 circuits won't work without these.

 

If you take a photo of your actual project and post it, we may be able to debug any wiring errors for you.

 

I think Dodgydave has nailed the fundamental issue. The circuit is flashing the LED at a rate that causes it to look constantly lit, due to the persistence of vision.

 

I cannot get it to work although I have rechecked it multiple times. Using an LED there is voltage at all times at pin 3, which I can vary with either pot, but not get it to switch.

What do you mean by it not working? What is it not doing?

The diodes should be more than fast enough for this app -- this is dinky slow for them (or any diode, I would think).

I don't understand what you mean by "Using an LED there is voltage at all times at pin 3" or, even more confusing, what you mean by being able to vary it with the pot.

Reading between the lines, is the following a more accurate description of what you did and are seeing?

You connected an LED (with a series resistor, I hope) to pin 3 and it is lit. As you adjust the pots you can make the LED brighter or dimmer, which you are interpreting as a varying voltage on pin 3.

If this is a good description of what you are seeing, then your circuit is working fine. What is happening is that the LED is turning on and off fast enouigh that your eye sees it at always being on. But, at any moment, it is either on due to full voltage on pin 3 or off due to low voltage on pin 3. When it is brighter, it is spending a higher fraction of the time "on" and when it is dimmer it is spending a higher fraction of the time "off". Hence, your pots are doing exactly what they are supposed to do, which is adjusting the length of time that pin 3 is high and low, respectively.

 

With the values shown the frequency is about 65Hz so the leds will be lit dimly, if you want to slow it down to make an led flash
, change the capacitor to 10uF instead of 100nF

Thanks for the help, it didn't occur to me that it was flashing @ 65Hz, but it makes sense. I'll make the changes and report

Geoff

 

Add some bypass caps.
Add a 0.1 uf ceramic cap from V+ to ground, and add .01 uf capacitors from pin 5 to ground on at least the 1st (left) 555. Some 555 circuits won't work without these.

Thanks so much, I'll try your advice and report results

Geoff

 

I think Dodgydave has nailed the fundamental issue. The circuit is flashing the LED at a rate that causes it to look constantly lit, due to the persistence of vision.

I'll let everyone know when I've got it going

 

What do you mean by it not working? What is it not doing?

The diodes should be more than fast enough for this app -- this is dinky slow for them (or any diode, I would think).

I don't understand what you mean by "Using an LED there is voltage at all times at pin 3" or, even more confusing, what you mean by being able to vary it with the pot.

Reading between the lines, is the following a more accurate description of what you did and are seeing?

You connected an LED (with a series resistor, I hope) to pin 3 and it is lit. As you adjust the pots you can make the LED brighter or dimmer, which you are interpreting as a varying voltage on pin 3.

If this is a good description of what you are seeing, then your circuit is working fine. What is happening is that the LED is turning on and off fast enouigh that your eye sees it at always being on. But, at any moment, it is either on due to full voltage on pin 3 or off due to low voltage on pin 3. When it is brighter, it is spending a higher fraction of the time "on" and when it is dimmer it is spending a higher fraction of the time "off". Hence, your pots are doing exactly what they are supposed to do, which is adjusting the length of time that pin 3 is high and low, respectively.

Thanks for the insight. I would think you are correct and I'll try all of the suggestions everone made. I'll report the results.

 

Hi Folks,
I built a second circuit on a breadboard and got the same results as the hard wired version. As per your collective advice I added an 0.1uf ceramic cap between V+ and ground first. It didn't change anything that I could see. I then tried an .01 cap between pin 5 and ground, that turned the LED off and nothing else happened after that, so I removed it. I got a 10uf tan cap and put it in, the led began flashing, quickly. The end product I want is a continuous 20 seconds on and 10 seconds off, but my pots on the breadboard are only 50k so I'll make the same (successful) changes on the hard wired version where I've got 100k pots. Thanks so much for ALL the advice.
Geoff

 

Hi Folks,
I did report some success with the info you provided, however I can't find it here. Adding a cap @ V+/grd didn't appear to do anything and the cap @ pin 5 turned the LED off. I changed the 100nf cap to 10uf as suggested by DodgyDave (thanks) and that started the LED flashing much slower. I've since tried various caps up to 100uf and each successive increase slowed the flash rate. Next job is setting the pots to get to 20secs on & 10 secs off, which is the point of the project. I'll report back when it's done. Thanks again to all.
Geoff