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tried making something like this Limit Number Of Pulses From 555 Or Similar but it doesn't work. I'm not sure what it needs. I'm trying to make the first timer put out voltage to the second one for about 6 sec then have the second one on for about 1 sec then off for .5 sec till the 6 sec runs out. thanks for any help. enter image description here

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  • \$\begingroup\$ The schematic looks fine. Double-check your wiring. \$\endgroup\$
    – Dave Tweed
    Commented Nov 7, 2014 at 23:40
  • \$\begingroup\$ R1 should be 55k. \$\endgroup\$
    – fisherr87
    Commented Nov 8, 2014 at 0:01
  • \$\begingroup\$ Yep. Naturally, when I looked at it the first time, I just read it as 55K and didn't give it a second glance. \$\endgroup\$
    – Dave Tweed
    Commented Nov 8, 2014 at 0:30
  • \$\begingroup\$ What is working? Are you getting a pulse out of the first timer? \$\endgroup\$ Commented Nov 8, 2014 at 0:36

1 Answer 1

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Your circuit looks okay. I redrew the circuit in CircuitLab so I could simulate it. Although you showed the circuit using two 555's, and I followed that, you can get both timers in one package as a 556.

Your timing diagram shows two pulses, but with 6 seconds, and a 1 second on, 1/2 off pulse from the astable, you are going to get three. So I left it at that.

I verified the values of the resistors and capacitors you chose (first substituting µF for F) using two calculators, one for the monostable and another for the astable). The timing for the monostable was right at 6 seconds, and for the astable at 1.47 seconds with a 67% duty cycle (0.98 seconds on, 0.49 seconds off).

I then changed the time for the monostable to 5.5 seconds instead of 6, since four time periods of 1.5 seconds equals 6 seconds, and depending on how accurate your timing is, you might get the beginning of another pulse from the astable right at the 6 second mark. At least I did in the simulation. So I used a 50K resistor for R1 rather than the 55K one. Obviously you can change these values with the calculators.

When I was done, I used the CircuitLab simulator. This is what I got:

enter image description here

As you can see, there are three pulses within the six seconds as expected. The first pule is longer, because as Dave Tweed points out, the timing capacitor of the second timer is charging all the way from 0 volts to 2/3 Vcc, while for the subsequent pulses, it's only charging from 1/3 Vcc to 2/3 Vcc.

So the circuit should work as it is drawn, as far as I can tell.

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  • \$\begingroup\$ The first pulse is long because the timing capacitor of the second timer is charging all the way from 0 volts to 2/3 Vcc, while for the subsequent pulses, it's only charging from 1/3 Vcc to 2/3 Vcc. That's why you have two time constants in the 555 datasheet: 1.1RC for monostable mode and 0.693RC for astable mode. \$\endgroup\$
    – Dave Tweed
    Commented Nov 8, 2014 at 14:25
  • \$\begingroup\$ @DaveTweed Thanks, I figured it was something like that, but it hadn't occurred to me that the timing for the first one would simply be the as for a monostable. Makes sense. I'll update my answer. \$\endgroup\$
    – tcrosley
    Commented Nov 8, 2014 at 16:17
  • \$\begingroup\$ Thank you! I thought I had it set to email me when someone answered I didn't get one so i thought no one did. I needed four pulses from the second timer my math was just off. when I turned power on I would get the second timer cycling with out a pulse from the first. I continued to work on it, finally i tired to disconnect the V+ to the second timer and use 3 from the first timer has the v+ in for the second and it started working just with 3 instead of 4 I changed R1 to 68k that fixed that. thanks for all the input. \$\endgroup\$
    – fisherr87
    Commented Nov 8, 2014 at 23:35

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