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Hi

I'm currently working on a cascade 555 circuit, both operating in the monostable mode. I also want to be able to control the pulse width with a voltage so am using applying 0-5v to the ctrl pin (The supply voltage is 10v). I've simulated the circuit and it seemed to run fine. However when I built it there seems to be an issue coupling the two timers together.

When the output of the first timer goes low and causes the capacitor to discharge through the first timer this should pull the second trigger pin to 0V. However in reality it's only going to about 2.6V which means that it will only trigger when the control pin is higher than ~5V (this is indeed the case). I've tried varying the 100k resistor to see if that would help, but that only seems to change the rate of recharge, not the initial voltage.

Does anyone know why this might be happening and how I can get it to drop further?

Thanks

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  • \$\begingroup\$ This is a canned comment to let you know that it looks like you're trying to implement something that is non-trivial with the power-hungry NE555 but is very easy for someone using a microcontroller. \$\endgroup\$
    – Marcus Müller
    Oct 1, 2019 at 10:38
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    \$\begingroup\$ Just an observation: don't put components diagonally on a schematic. Occasionally it's useful, such as diode/capacitor trees, but here it isn't, I'm afraid. \$\endgroup\$
    – TonyM
    Oct 1, 2019 at 10:54
  • \$\begingroup\$ Please also add reference designators. It makes addressing components easier and most inportant: unambiguously. \$\endgroup\$
    – Huisman
    Oct 1, 2019 at 14:42
  • \$\begingroup\$ How are you setting the control voltages? If you are using resistor dividers, you will run into trouble due to the fact that the CV pi is connected directly to the internal resistor string. \$\endgroup\$
    – Caleb Reister
    Oct 1, 2019 at 14:44
  • \$\begingroup\$ The other potential issue I see is that you are trying to AC couple the output of the first 555. \$\endgroup\$
    – Caleb Reister
    Oct 1, 2019 at 14:46

1 Answer 1

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Thanks for everyone's help and advice. Ive added a voltage divider as per above which seems to have resolved the issues. The voltage control is from a PWM on an arduino filtered through an active low pass filter. Everything is now working as expected.

Thanks again

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  • \$\begingroup\$ I'm a bit late, but if you already have an arduino in there, adding a timer chip seems to be a waste. Simply program the arduino to do what you need. \$\endgroup\$
    – mmmm
    Mar 8, 2021 at 0:52

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