Asking this question has really been a helpful lesson for me, teaching new aspects of capacitors I need to know. Thanks to all of you who answered.
I have use back to back caps in many projects.
Glad you told me. Now I know I can trust the "back-to-back" technique, knowing the person who previously told me about it wasn't making an unfounded guess.
Use a LM339
That's an excellent idea, by using an LM399 comparator instead of an op amp, a feedback resistor from output to input could set the range between positive going trigger and negative-going trigger, and then send just a "1" or a "0" to the software; instead of the way I have it now, sending the software analog data in a string of bytes for the sketch to accomplish the same tasks as the LM399 could have already done.
bias the center lead with
a high ohms resistor. Such as, if the + leads together place a 1 meg resistor to
the + power lead. The capacitors will always remain properly biased.
Now that's an extremely smart idea I'd never thought of. If I were staying with the electrolytics, I'd use it!
Why use polarized caps in the first place? 4.7 uf ceramics are readily available and dirt cheap.
D'OH! I feel embarrassingly dumb, not to have discovered this on my own. I saw the many polarized versions online and assumed that's all that existed. I would not have needed to post this question. But thanks to you, I've now ordered ceramics in both 4.7uf and 10uf values. (And they really are cheap.)
I suggest to look for another [circuit] with proper biasing. You shouldn't need unpolarized caps.
take a look at http://www.learningelectronics.net/circuits/images/heart-rate-monitor-circuit-diagram-2.jpg..., which is an AC-coupled amp [for heart monitoring], biased at the midpoint of the power supply, and does not require a polarized cap.
Thanks. I'll build and test yours example, hoping it really will work better for me. Also the website it's from, LEARNING ELECTRONICS, seems an outstandingly interesting place to explore.
High value ceramics are often unsuitable for analog coupling capacitors as they are...
wide variation between devices...
Perhaps ... use plastic film...? Plastic film don't age like electrolytics.
All really good points. I'd forgotten about "microphonics". I first discovered that when I brushed my finger across a ceramic cap, and heard it in the speaker almost like I'd brushed across the actual microphone!
I've now looked up info on "plastic film" capacitors, and they are just too big! This project is a tiny item to be worn.
I'm glad you confirmed electrolytics don't last. I worked in a computer repair shop where it seemed we were having to replaice a lot of blown electolytics from motherboards. So, they won't be my choice (where I have a choice).
All said and done, I think ceramics are the best choice for this circuit, as I only need it to see the heartbeat frequency, and report when a beat occurs.
Below is the circuit I'm actually using. It is better than some mentioned above because it filters out noise very well, which is a must for Arduino analog input sampling. In designing it, I varried part values until I got the best output; highly sensitive and clean.