Timing Problem with First 555 Timer in Double Astable Circuit

Thread Starter

Matthew464

Joined Jan 22, 2017
13
I am trying to make a double astable timing circuit like this one I made on LT Spice. It works fine on the computer, but when I made it on the breadboard the timing is all wrong. The power supply is 9v and the first timer triggers the second timer through the reset pin. After measuring the voltage at the trigger pin of the first timer, I found that it was going from 3V to 3.6V, when it should be 3v-6v. I tried testing the first timer by itself without connecting it to the second timer and it oscillates normally with the trigger pin ranging from 3v-6v. But once I power the second timer along with the first, it starts messing up again. After checking all my connections and even moving my components to another breadboard it still doesn't work. I then tried to see if the problem was my power supply. So I left my power supply connected to pin 8 of timer 1 and disconnected it from pin 8 of timer 2. I then powered timer 2 with a 9v battery and the circuit worked...So I'm guessing the problem lies in my power supply. The power supply I have is a 1.5 Amp regulated switching power supply that has worked fine so far for me(Although I did have a past problem with it in a similar 555 timer circuit). There also may have been a few times where my power supply may have been shorted out for a brief second or 1-3 seconds. What do you guys think? Do I need a new power supply?

Here is the link for my power supply https://www.amazon.com/Velleman-PSS...=8-1&keywords=velleman+switching+power+supply
 

Thread Starter

Matthew464

Joined Jan 22, 2017
13
**Update**
I tried powering the entire circuit with a 9v and it didn't work so I guess it's not my power supply.

Sorry, I forgot to upload my circuit. Here it is:
 

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AnalogKid

Joined Aug 1, 2013
9,185
So, at a 1.2 Hz rate, you want to cycle an output oscillating at a 9 Hz rate? Is that it, or do we have to guess some more?

Most switching power supplies require a minimum load current to stabilize the internal magnetics. They are not general purpose supplies for very low loads. Unfortunately, this is rarely documented in a spec listing. I recommend debugging the circuit with a separate 9 V battery for each 555. Once that works, change to a single battery for the entire circuit.

ak
 

Thread Starter

Matthew464

Joined Jan 22, 2017
13
So, at a 1.2 Hz rate, you want to cycle an output oscillating at a 9 Hz rate? Is that it, or do we have to guess some more?

Most switching power supplies require a minimum load current to stabilize the internal magnetics. They are not general purpose supplies for very low loads. Unfortunately, this is rarely documented in a spec listing. I recommend debugging the circuit with a separate 9 V battery for each 555. Once that works, change to a single battery for the entire circuit.

ak
 

Thread Starter

Matthew464

Joined Jan 22, 2017
13
I tried two 9v batteries and one of them worked. One was 8.2v and the other was 7.7v. The lower one worked. I also tried putting a 100uF cap from Vcc to ground and that fixed the whole problem. It now works with my power supply. I read that a decoupling capacitor fixes many 555 problems, I just never tried it
 

AnalogKid

Joined Aug 1, 2013
9,185
One of the problems with using a simulation package as a design package is that it forgives things like no decoupling caps. And the 555 is particularly bad at putting junk on its own power rails.

ak
 
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