We are going to build a telegraph which can communicate to an other telegraph. One of our communication systems exists out of a bell. The idea is that the sender can press a button and the receiver will hear one single ping. This bell will be operated by an electromagnet. We also want to send a few pings after each other.
12 V and ground are fed by a flat 6 pole RJ12 cable. There is one pole for 12 V and one pole for ground.
There are two features I would like to implement. I want to add a capacitor discharge unit to power the coil with as purpose not drawing too much current from the 12 V line. (we would see LEDs flicker)
The CDU on it's own can power the coil. But if one is too send three pings, he might deplete the CDU too fast if hold the button too long. And the hammer might be touching the bell too long
For the second feature I would like to add a capacitor in the receiving line to let the button switch cause a pulse which can trigger an opto coupler.
The operation in this circuit in order:
One presses the button at
bellTx, this causes a pulse on the receiver which triggers the optocoupler for a short time.
When the optocoupler is active, the MOSFET Q3 will be conducting which should sink the coil to ground.
When that happens the CDU should discharge the coil for as long as the pulse is active or the CDU still has energy.
What I want to know:
Did I make any obvious mistakes in this circuit? I was not sure if I have to tie
bellRX to ground via a resistor as well?
How can I calculate the on time of the opto coupler when I press the the switch near
I do not yet know how long I should switch and how much energy I am going to need. I do can experiment with the opto coupler pulse, but I'd like to know how to calculate the resistor and capacitor values.
I figured it would also be a good idea to add a potentiometer somewhere in the pulse line. This would give me more control over the pulse time.