# Op-amp astable multivibrator with single supply

I want to create a simple circuit to flash a LED with one op-amp. I already simulated the circuit (astable multivibrator) but there is one problem with the supply. I don't know what to change so that it works for single supply. However, when I use dual-supply it will burn my LED when placed like on the circuit below. The output will oscillate between 10V and -10. Unfortunately, -10V is below the max. allowed voltage for an LED (which is maybe 5V?).

I have two questions now:

1. What would I have to change when I want it to work with dual-supply (currently it would kill my led)?
2. How do I need to change the circuit to get it work with single supply?

Thank you!

• You want to know how to get it to work for both dual and single supply while it currently works for neither?
– Mast
Commented May 26, 2020 at 5:38

What would I have to change when I want it to work with dual-supply (currently it would kill my led)?

Put a reverse protection diode across the LED (or maybe a reversed LED of a different colour to get a nice flashing two-colour effect).

How do I need to change the circuit to get it work with single supply?

Try this: -

The frequency might need a bit of an adjustment if it's critical.

• Great, thanks. For the dual-supply case: would it also be possible to connect the LED to VCC (instead of GND) + increase the resistor in front of it? For your solution I would need a special diode which doesn't break on -10V, right? Commented May 22, 2020 at 15:25
• @Spacey3 - " I would need a special diode which doesn't break on -10V, right?" Nope. The new diode will "point" in the opposite direction as the LED When the LED is on, there will only be 2 - 3 volts across it (since the LED is on) which will not stress any diode you can buy. When the op amp is putting out -10 volts the new diode will be turned on, and will only have about 0.7 volts across it. What you do have to worry about is the op amp. a 400 ohm resistor will draw more than 20 mA, and not many general-purpose op amps will be happy trying to do that. Commented May 22, 2020 at 15:51
• @Spacey3 you can connect the single LED load to the positive rail and not worry about the reverse protection. Make sure LED anode is towards the positive end. Commented May 22, 2020 at 17:01
• I still haven't figured out how the reverse diode should work... do you mean a second diode parallel to the LED? Because when I add another diode above or below (as a serial connection) the led wouldn't light anymore at all because each diode blocks one "wave" Commented May 22, 2020 at 18:34
• Yes, a parallel diode. Sorry I wasn’t clearer. Having said that I did say “across” and that implies parallel lol. Commented May 22, 2020 at 18:55