# Sound reversing robot car

can someone tell me how the sound reversing robot car works

I’m not sure what part of how it works you’re interested in, so I’ll try to cover all the bases.

Elenco has the manual for the sound reversing car on their website, you can look at it here.

I don’t own the kit myself, but from the manual it looks like the front wheel swivels freely 90 degrees between two stops. Going forward the wheel tends to point straight ahead, but in reverse the wheel drags and swivels around so the car turns. There is one DC motor connected through a gearbox to the drive wheels, so all the electronics have to do is drive the motor forwards or backwards.

Looking at the circuit diagram on the last page of the manual, the motor is connected to an H-bridge made of 4 discrete transistors for forward and reverse power. Two more transistors are used as dedicated logic inverters, so that the bridge is never shorting the battery!

The rest I’m not 100% sure about, but I’ll take a stab.

The condenser microphone is basically a capacitor, whose capacitance fluctuates with applied pressure, i.e. sound waves. In a quiet room, it quickly accumulates its full charge. When a loud noise (a clap, or the car hitting a wall) initially compresses the microphone, it’s capacitance rises much faster than it can accumulate more charge, and the voltage drop across the microphone decreases proportionally.

From just looking at the schematic, I’m not clear on precisely how the rest of the circuit works. The cascade of three transistors seem to be forming an amplifier, with the various resistors and capacitors forming an RC circuit with some short time constant. It looks like when the microphone is compressed by a loud noise, the voltage spike it produces is amplified and either charges or drains C3, which flips the direction of the H-bridge, making the car back up. When the microphone returns to its normal state, the capacitors slowly recharge (or discharge, again, I’m not doing a full circuit analysis here) and the H-bridge resets to its initial forward state.