In a previous article, I wrote about reading to a parallel port using the signal port. What I needed to read in was the signal from a photodiode. The following is a decent method to amplify the photodiode signal using common IC chips.
This is a schematic of a photodiode amplifier that I used for an earlier project. It produces enough current to trigger a relay that connects the 'signal pin' to a 'ground pin.'
The components are as follows:
- LED or any light source, mine was a laser
- 3 Pots (Potentiometer) or Resistors; R1 = 3.3K, R2 = 104.6K, R3 = 312.2
- Opamp, mine was a UA741
- NPN Transistor, mine was 2N3904
- IC Relay, mine was a Hamlin HE721C0500
- Source: You'll need a source with at least 9 Volts. Note that you'll need a source that also includes a negative voltage and ground. You can substitute +9V or my +5V and -9V for my -5V. However if you also want to substitute my +12V for a +9V, then you may have to tweak R3.
*Important note: I take no responsibility for any mishaps.
*This Diagram/Schematic can be found at www.datasheetcatalog.com.
My photodiode can only provide at most .9V (I think). The relay I'm using requires more current than my photodiode could provide. I needed the relay to trigger the switch inside of it and allow the 'signal pin' to be grounded; this would then result in a low on my signal pin. And as indicated from my previous article; I need the signal pin to be grounded so I could read it as data.
Some may say; why didn't you just use the transistor. Wouldn't it be easier? My answer is nah! Due to the fact that the base of the transistor needs a decent current to allow Vbe to go to saturation, I used a opamp to boost the signal and then tie the output to the base of the transistor. I used 12V at the collector since I didn't have 9V.
If you have any questions, email me at firstname.lastname@example.org.
- 8th August, 2006: Initial post