I'm trying to build an isolated circuit that can detect high-voltage pulses within a certain duration window and active a low voltage side accordingly.

The low-voltage side should activate for:

  • Pulses > 18V and >30ms duration (with 180mA maximum current)

The low-voltage side should not activate for:

  • 100 uA leakage input current
  • A pulse of 29V with 0.1ms duration

Additionally, there is a hard capacitance limit of 300pF on the input side.

What I've come up with is the following circuit using the TI ISOM8110 opto-emulator:


simulate this circuit – Schematic created using CircuitLab

This solution works, but is very part count inefficient. Is there a way to solve this more elegantly?

Also, is there something to consider using a capacitor between the base-emitter of the PNP transistor like this? I've not seen it used like this, but I can't think of another way to delay the on-switching sufficiently.

  • \$\begingroup\$ For the low voltage side, what voltages do you need for it to register a logic LOW/HIGH? (eg an software controlled ADC is very flexible, while an logic IC can be more strict) Do you have other voltage rails available that you can use to reduce part counts \$\endgroup\$
    – Ferrybig
    Apr 16 at 9:13
  • \$\begingroup\$ @Ferrybig, this output drivers a NPN transistor (not shown), which effectively grounds the receiver pin, so effectively I have a open-collector output stage. The circuit is fully analog and +12V is the only voltage rail available. \$\endgroup\$
    – Malte
    Apr 16 at 11:55
  • \$\begingroup\$ I have a thought that could meet this. But I need to think more closely about your specs, first. So for now I'll +1 the question. \$\endgroup\$ Apr 16 at 16:08
  • \$\begingroup\$ Emitter and collector of the coupler are swapped. \$\endgroup\$
    – Jens
    Apr 16 at 20:08

2 Answers 2


Maybe you could use a 16V zener diode as voltage comparator and RC for pulse duration. A 4k7 + shottky quick discharges the cap in case of short pulse so the cap will be ready soon for next pulse.

enter image description here


One approach can be using an p-channel mosfet instead of BJT transistors. Common P-channel mosfets have higher tresholds than BJT's, so you can use a smaller sized capacitor for the treshold detection

enter code here

Online simulator link

This circuit uses 6 extra parts on the low voltage side, it is possible to get rid of the NPN BJT, this transistor makes the circuit turn of way quicker so repeat shorter pulses do not trigger the low voltage side.

If you step into the domain of IC's, you can even same more on your part count. For example, the following uses a 555 timer to reduce the part count on the low voltage side to just 4:

enter image description here

Online simulator link

Note that that you might need to tune the resistors and capacitors for the correct trigger levels. I just tuned them to have them detect a 30ms pulse, but not a 10ms pulse. Depending on the size of your "don't care" range of the allowed inputs, you have options. Note that not every 555 timer supports 12V supply voltages


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