Begin Charging Delay Circuit
When the .Net Serializer tells the recharge circuit it is time to restart, we want a delay between the time the multiplexer is configured for the proper time and the time the reset pin is set on the timer to start the timing. To do this, we’ll create a delayed “one-shot monostable multivibrator” (fancy name for something which can do a single timed pulse) using a couple of D Flip Flops. The basic idea for the circuit comes from here, but is modified to add a delay.
The left hand flip flop simply converts a slowly sloping capacitor charge voltage into a nice fast edge when it hits a certain voltage. This is needed because we need a nice fast edge to trigger the clock input on the second Flip Flop (the clock is edge triggered and the SET input on a flip flop is just voltage triggered). The second flip flop then turns on and stays on for a certain period of time. The timing for the delay and the length of the pulse could be different if we used different values of R and C for the circuit, but we don’t care so we’ll keep it simple and use the same values.
The equation for charging a capacitor is: Vcap = Vfinal * (1 – e^(-t/(RC)))
Solving for R: R = -t / (C * ln(1- (Vcap / Vfinal)))
- Assume C = 1uF because it is available
- Vcap = 7V (the trigger voltage of the clock)
- Vfinal= 9v (power supply voltage)
- t = 200msec (the length of time we want to wait for the multiplexer to get configured)
R = -200e-3 / (-1.5041e-6)) = 133K. We don’t have that size, what about 100K?
Solving for t = – R * (C * ln(1- (Vth / Vfinal))). t = – R * -1.504e-6. Using 100K gives us 150msec. Using 220K gives us 330msec. That is better.
