In Phototransistor I talked about one possible phototransistor lab, that looked at the response speed of a phototransistor, as a function of the load resistor. I rejected that last year as insufficiently interesting for bioengineers.
The lab for phototransistors that I used last year was a “tinkering” lab, where I tried to get the students to play with the hysteresis oscillator that they had built, modulating it with light (see Idea for phototransistor/FET lab). I didn’t think that it was a very successful lab (see Tinkering lab reports show problems), and I’d rather have a lab that seems more directly “bio” oriented.
One lab I’ve not given in class, but have played with a lot at home, trying to find something that works at the right level of complexity for the students is an optical pulse monitor:
- Pulse detection with light
- Giving up on light-based pulse sensor
- Optical pulse monitor with little electronics
- Digital filters for pulse monitor
- Failed attempt at pulse oximeter
I played around with the idea some more last week, using a transimpedance amplifier to convert current to voltage (as in Colorimeter design—weird behavior). I can easily get enough gain to see pulse for a 700nm LED shining through a finger, but I listed the “brighter” LED red diffuse 3mm 625nm WP710A10ID part for this year’s parts kit, so I need to test with it (or with LED IR emitter 5mm 950nm SFH 4512). Because I’ll be making the mechanical part of the pulse monitor for the students, I have to know whether a 5mm or 3mm LED will be used.
Because oxyhemoglobin has its lowest absorbance near 700nm, I expect that switching to either 950nm or 627nm will greatly reduce the signal, needing an extra gain of 5.
To connect the LED and phototransistor to a breadboard, the leads need to be extended:
In order to help me remember which side has the phototransistor and which the LED, I color-coded the leads differently (yellow wire for LED anode, green wire for phototransistor collector), and used colored electrical tape to hold the optoelectronic parts in the block (red tape for the LED, blue tape for the phototransistor—matching their package colors).
I did manage to get the pulse monitor working sometimes, but it seems to be excessively finicky—I need very high gain with careful setting of the bandpass filter parameters to get a signal. The biggest problem is that the second stage of the amplifier, where I do the high-pass filtering to remove the DC component and slow drift, can end up getting saturated. Because of the high impedance of the feedback resistor, the output stage takes a long time to recover from being saturated. Saturation is a frequent problem with high-gain amplifiers, but I’m not sure I want students dealing with it on this lab.
The combined gain of the two stages at 1Hz (about the frequency of my pulse) is around 132MΩ, and the output is still only about 0.25V, so the fluctuation in the input current must be around 2nA. That’s not as small as the signals in a nanopore, but it is small enough to be troublesome.
I tried a different set of components that gave me a gain of about 240MΩ at 0.9 Hz, and that amplifier started clipping the output, swinging from around -0.8v to +1.6v.
After the first stage (with a gain of about 1.7MΩ at 0.9Hz and 5.6MΩ at 0Hz), I see about a 10mV swing on top of a DC signal of 0.6 to 0.8v (with considerable drift). That implies about a 6nA signal at 0.9Hz, while the DC signal is about 125nA. The magnitude of both the DC and the AC component varies a lot, depending on which finger I use and how firmly I press the finger against the sensor. I can pretty consistently get 2–9nA of AC on top of 100–150nA DC. I think that good corner frequencies for the low-pass and high-pass filters are around 0.3Hz and 30Hz. By making the gain of the transimpedance amplifier as high as I can (without saturating with the DC signal), I can keep the gain of the second amplifier low enough to avoid the problem of saturation in the second stage, and the pulse monitor can detect the pulse within 5 seconds.
Another option is to make the first-stage amplification be a logarithmic transimpedance amplifier, rather than linear one, by using a Schottky diode as the feedback element instead of a resistor. But that is getting well outside what I’m comfortable assigning as a design exercise to the Applied Circuits class. I tried it anyway, but the signal from the log amplifier was too small: a 10% variation in current only results in a 2.4mV change in the output of the log amplifier, needing a much higher gain than my second stage currently provides.
While the 700nm LED provides a stronger signal, the 627nm LED works well enough, and a 2-stage transimpedance amplifier is reasonable for the students to design. I probably want it to be a 2-day lab, though, with the low-pass first stage designed and tested for the first day, then the high-pass second stage added to solve the problem of DC offset and drift. That will require reworking my schedule, as I only allowed one day for the lab in the current schedule.