The first interface is a single axes for the type of control often referred to as Wig-Wag centre is off, move the input pot forward to move forward and backwards to reverse.
This one is intended for use with 4QD’s NCC or Pro-120 series controllers which use a 9v internal supply, so the circuit is powered from this. However the supply voltage is not critical.
On the left is the 10K pot with a 10K in series with each leg, so that swinging the pot from maximum to minimum will swing the wiper from 3v to 6v. The centre position will be 4.5v, with a ± 1.5v swing. R3 and R12 are normally fitted but, on 4QD’s circuit board, R10 and R2 may be fitted instead to reverse the action of the potentiometer.
R1 and the preset are optional and are for centring the input. However that is always best done by mechanical adjustment so the mechanical centre and true electrical centre coincide. These components also mean that, if a wiper wire to the input pot breaks, the circuit does not go to full speed but remains at zero (centre).
The IC used is a quad op-amp such as LM324. The first stage simply buffers the pot wiper. So we get 4.5v ±1.5v on pin 7. This is fed to the second and third sections of the chip which form a precision rectifier circuit. To understand this, consider only the base-emitter junction of the PNP transistor, which is used as an ordinary diode.
Pin 10 of the IC is biased at 4.5 volts (the zero point) via R14 and R15. If pin 7 rises above this, pin 8 will fall below 4.5v and the transistor diode will be reverse biased, so you can ignore pin 8/9/10 section of the IC for positive swings.
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