Need to drive a bi-color LED in one of your DIY projects? I needed some of these for the added LEDs in my Dual Benjolin project. I had a circuit in mind, one used by Ken Stone (CGS) in his version of the Serge Smooth and Stepped Generator. Here’s a version of that circuit, provided by J3RK in a discussion of LED driving on the ModWiggler forum.

I had gone to that MW discussion to ask if the 1K resistor on the input created a 1K load. Not surprised to find that yes it does. This circuit will work best if preceded by a direct op amp output. But I needed a circuit with a higher impedance input load.

Mungo Enterprises replied with the following schematics, the first with specific values and the second with formulas. D1 and D2 in the following can be separate or combined two-lead LEDS. The advantage of this circuit is the higher impedance input of 220K ohms.

I set about breadboarding the Mungo design, looking for a maximum LED current of 20ma, which is the specification for the LEDs I used, Kingbright WP7113SRSGW, unfortunately obsolete. I had bought some newer ones, Kingbright WP57EGW, but I preferred the red color of the former. Here’s the circuit I ended up with to use in the Benjolin project. I measured the maximum current through the LED to be 17ma with a 5 volt input.

This is the Mungo circuit with selected values. Where he has 220K, I think that anything between 220K and 470K would work fine. I used 390Ks simply because I had a lot of them. Where he used a 10pf capacitor, I used 47pf. Again, anything in that range would work. And I used a 20K instead of 22K because I had a lot more 20Ks on hand.

As for the 330 ohm resistor, that’s the one to select to set the max LED current. The lower the value, the more current. A 1K value here might be better if the voltage being monitored can be patched in. In my usage, the input voltages are limited to +/- 4.5V.

Even though the Mungo design uses two more resistors, it saves having to add an additional op amp buffer in front to drive the 1K load.