|Simplified LT-Spice schematic drawing|
A pulse of about 1ms is fed through the DC-blocking C1. The pulse in the actual design comes from the IR-diode, which picks up reflected IR from the IR-LED.
U1 is simply a non-inverting amplifier which is AC-coupled using C2. C2 makes the gain roll of to unity at DC.
|The big pulse is from the IR-LED, for triggering purposes. The small pulses are from the U1+ node, a high pass filtered pulse.|
|And the same filtered pulse in LT-spice. Very similar to reality.|
U1 amplifies the positive going pulse to look like this:
|Amplified pulse, almost rails the output of U1.|
|Amplified pulse of U1, looks very similar when simulated.|
This is an emitter follower with a diode on the output and a bleeder resistor to ground. U2 can only source current to node U3+. After U3+ is charged,the voltage is kept high by C3 and R4 for a while.
|The small pulse of node U3+ is not what we expect.|
|The simulation shows what I expected. A high step that slowly bleeds out through R4.|
So the problem is probably in node U3+. Perhaps a bad connection to the cap or a bad cap. I'll look more into it. In the following sections I'll just explain what is supposed to happen and show the simulations.
Is a slow integrator that keeps the output high while U3+ is discharging. If the original incoming pulse (at U1+) is smaller than some threshold, U3o won't have time to integrate to any significant level before U3+ is discharged. U3o then stays low.
|U3o when it has had time to integrate to a high voltage level.|
This is a comparator. If U3o is over 3V, U4o drops to it's lowest level. If U3o is below 3V, U4o kicks up to it's max output.
|Complementary outputs of U3 and U4|
U3 and U4 has complementary voltage levels at their outputs and the are used to drive the H-bridge to one of the robots motors, making the motor reverse for a while when the robot receives a big echo from it's IR-receiver. This makes the robot back out and spin 90 degrees before continuing forward.