In our earlier post about driving the LED sources we wrote something like “creating triangular wave is straight forward”. But it was not. We struggled to create reasonable triangle wave source using a single side (+5V) op-amp circuit. The shape of the wave was ok but we just could not make it stable and found it drifting into rails. So, until we come up with a workable circuit we will use pulse wave to drive the LEDs.
The circuit we will be using to drive the LEDs for now is here:
Some short notes about this circuit:
- Port “Control In” is input connection for the pulse wave. D1 regulates the level of the high state of the pulse. R2 adjusts the regulated height of the pulse. Both +3V and +5V pulse can be used.
- A simple low pass filter was added to remove the high frequencies from the state change of the pulse.
- We connected the LED (connector) between R2 and GND. This way we can use a co-axial cable with shield connected ground level. More about benefits in this article [1].
- If pulse frequency must be significantly higher, a high value resistor can be added across the LED to help voltage return to LOW level.
- The op-amp was not specified but it could be AD820 or OPA344 or any that works well with the single supply +5V.
References:
[1] “An improved circuit for control of LED luminance”, https://link.springer.com/content/pdf/10.3758/BF03203658.pdf
Experimental Photonics 