oscillatorrelay
Is it possible to create a very simple oscillator using relays to cause the current to quickly switch back and forth between two lines (or even pulse on one)? It seems like it's almost possible, but I can't figure out a way.
I think if I could design a switch that stops current when current is applied, it would be possible.
I can't figure out how to wire it up exactly. Here's what I'm looking at
You want "form C" capability. Without a lot more information on what limitations you're willing to accept, it is difficult to answer this, but I'll take a crack at it.
Since you are willing to accept an N-channel MOSFET with source grounded as your Normally Open contact, and the Common is therefore grounded, I'll assume you can accept the Normally closed also with the Common grounded. Keep in mind that both loads have to be from some positive voltage relative to ground). The below schematic requires a supply Vg for the N.C. MOSFET gate.
simulate this circuit – Schematic created using CircuitLab
Another way is to simply buy an opto-isolated MOS relay that uses a depletion-MODE MOSFET for the N.C. contact. Using a depletion-mode MOSFET is about the only way to get a closed contact with no power applied.
It's extremely difficult to find a microcontroller on the market these days that does not have some kind of counter/timer module built into the chip — usually they have several, because they are so universally useful. And unless you pick a very small, low-power µC, it will have no problem counting at 25 MHz.
If the hardware counter does not have enough bits, use it as a prescaler and do the rest of the counting in software. Use the counter overflow interrupt to increment a software counter, and combine the results as needed.
So, just connect your oscillator to the counter clock input, and configure the counter to free-run. Connect the GPS 1 PPS signal to an interrupt line. Each time the interrupt fires, read the counter. Subtract the previous reading from the current reading to find out how many oscillator cycles have elapsed in the current GPS second (accounting for roll-over, of course). Use this value to compute the next control voltage update.
And you will want to do this every second. Trying to average the count over hundreds of seconds may seem like a good way to get higher precision, but in fact, it will make the system very slow to correct itself after external perturbations (e.g., temperature and voltage changes).
Best Answer
If you feed the coil via a normally closed contact of the relay, as soon as power is applied the relay will open-circuit its own power feed and turn on and off at possibly tens to hundreds of times per second.
This can be slowed down by putting a large capacitor across the coil; as soon as power is applied and the relay energizes, power bocomes lost but the capacitor is charged and holds the relay energized for a little bit more time. The bigger the cap the more it holds the relay energized and slows down the frequency of oscillation.