0
\$\begingroup\$

As an amateur to breadboarding I'd been learning about transimpedance amps and photodiodes earlier this year for a project. Eventually I picked up a phototransistor (3DU5C) at a hobby shop, mistaking for a PD, so I learned a bit about them and found this circuit on physics portal after failing to get any readable voltage from a common-collector circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

The first time I tried the circuit using Arduino as a voltmeter I'd forgotten to hook up the Vcc- pin on the op-amp to GND, but the transistor responded to light (4.18V at ambient light and ~4.75V with a phone flashlight. When I noticed it, I completed the power supply to the OA and it started reading 5.00V consistently. Hooking up one power pin to different ports produced other various ranges, the highest being the Vcc port to GND (1.35-4.07). The transistor also seems to work with no power supply to the OA at all. Additionally, all cases seem to be switch mode. Any idea why this is, and how could I change this circuit to active mode?

|improve this question
\$\endgroup\$
  • \$\begingroup\$ Are those the actual part numbers on your schematic? The RCA4558 is not at all suitable. \$\endgroup\$ – Kevin White Aug 31 '17 at 3:06
  • \$\begingroup\$ @KevinWhite I ordered an LM358 but they sent me the RC. I'm using it until I can get my hands on an LM358, but if there's something wrong with that too could you tell me about it? I'm still new to choosing the right OAs \$\endgroup\$ – Alex Liu Aug 31 '17 at 3:22
  • \$\begingroup\$ Please include the power connections to your op-amp in the schematic. Are you using a single supply, or dual supplies (for example +/-5 V)? \$\endgroup\$ – The Photon Aug 31 '17 at 3:37
  • \$\begingroup\$ These OA's need a split supply, then use collector for input and emitter to Vee so negative current results in positive output. you may need a pot from Vin- to Vcc (+) in 50k range to null balance the bipolar output \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 31 '17 at 3:46
0
\$\begingroup\$

Your op-amp is not doing much at all. The RC4558 requires a negative supply voltage of at least -5V. Furthermore, its output voltage cannot swing close to the DC supply rails, requiring a volt or two "overhead". If you intend to use an op-amp, choose one that can work with low supply voltages, and one that has "rail-to-rail" output swing.

Your phototransistor is supplying photo-current through R1 (10K) to the microcontroller input pin. The op-amp is likely acting as a resistance to ground, so that voltage is less than the positive supply voltage.
You might be better off with this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab Even with a negative supply voltage going to pin 4, your circuit would produce a negative-going output voltage when the photo transistor sees light: not compatible with a microcontroller expecting to see a voltage between 0V and its supply voltage.

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ Would an LM358 be suitable? Additionally, I want to be able to use the transistor with an Arduino, so how should I modify the circuit so that it is compatible? \$\endgroup\$ – Alex Liu Aug 31 '17 at 4:31
  • \$\begingroup\$ LM358 can work with a single +5V supply (good), but its output isn't rail-to-rail...you lose over a volt at the top end of its range. And its input common-mode range means that input voltages shouldn't extend into the top 1.5V region. It is better than RC4558, but still not rail-to-rail. I don't know for what purpose you include an op-amp, so a suitable circuit eludes me. \$\endgroup\$ – glen_geek Aug 31 '17 at 10:20
  • \$\begingroup\$ From what I understand you're saying the schematic you drew isn't compatible with an Arduino - with or without an amp, is there a circuit which would put the transistor in active mode with a readable Arduino signal? \$\endgroup\$ – Alex Liu Aug 31 '17 at 15:36
  • \$\begingroup\$ @AlexLiu You misunderstand: the two-component circuit shown is very compatible with Arduino, operating from a +5V supply. Its one minor limitation is that it loses linearity for large photocurrents that causes Vout to rise close to +5V. If you need less sensitivity, you can always reduce R1, lower than 10k. \$\endgroup\$ – glen_geek Aug 31 '17 at 18:39
  • \$\begingroup\$ In that case, could you take a look at this setup? imgur.com/a/stIZ2 The output goes to A0 and all the +/- power rails are connected to 5V and GND, respectively. The serial is reading zero no matter how much light is shone on the transistor. \$\endgroup\$ – Alex Liu Sep 1 '17 at 4:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.