RCX IR Rx Circuit There is surprisingly little to the receiver circuit. The data output from the receiver device goes straight to the processor. The +5V pull-up circuit is common to the receiver and the sensor ports. The 3Kp transistor and the 22k resistors are also on the sensor port circuit. I've added another feed to the processor that hangs off the pull-up. This might be a signal that tells the processor that both +9V and +5V supplies are present, which might tell the processor to shut down safely if either supply were interrupted. -------------------------------------------------------------------------------------------- RCX IR Tx Circuit With the voltages as shown, the RCX is not transmitting. The IR LEDs are powered by a 600mA current sink. I haven’t measured for waveforms yet - the intermediate voltages may be averages of pulse trains. The lower transistor input from the processor is turned off by being pulled up towards +5V (PNP transistors require a low voltage on the base to turn on) The CDC transistor has a gain (hfe) of 250, hence the -25 suffix. This circuit was a bit more difficult since so many tracks seem to go straight to the processor. If I can glean any more from the H8 data sheet, I’ll add it later. -------------------------------------------------------------------------------------------- RCX Power Supply Circuit The 11.8V AC (from a 9V train adapter) is rectified to about 15.5V DC, smoothed by the capacitors. Presumably 6 x 330uF was the best fit within the case for the total capacitance required. The 5V regulator is used as a reference to set the current through the 22k resistor, via the right hand 3Kp transistor. The left hand 3Kp transistor receives a proportion of the 9V supply. This tells the MOSFET how much voltage to drop in order to keep the 9V supply stable, dropping about 6V with this AC supply. This will vary with other supply voltages. If three motors were being powered, the 11.8V AC would drop towards 9V under load. This would cause the 9V rail to drop, but the MOSFET compensates by dropping less voltage itself, keeping the 9V supply stable. I don’t think there is any switching in this supply, though that is what the MOSFET is designed for. The maximum MOSFET regulation is 6.8V, set by the zener diode, so the maximum safe DC voltage is 9.5 + 6.8 = 16.3V, so the maximum AC input is (16.3 + 1.2)/1.414 = 12.4V AC, which is why the maximum says 12V on the case. With a 9.0V AC input, the rectifier produces (9*1.414)-1.2 = 11.5V. Assuming 9.5V for the 9V DC supply, the MOSFET drops 2.0V. -------------------------------------------------------------------------------------------- RCX Sensor Port Circuit The readings for active and passive sensors basically differ by the fact that the 15.38mA current source is only used for the active sensors. All three sensor ports and the IR receiver are powered from the same transistor pull-up to +5V. I think this transistor may implement a delay function when the RCX is switched on, to make sure the processor is powered before any voltage appears at its ADC ports, so as not to damage them. The voltage at the port, to power active sensors, will vary depending on the supply voltage (mains or battery) and the current drawn by the sensor. It is over 9V with mains supply but may be as little as 7.6V with 9.0V batteries (one diode drop in supply circuit, 0.6V for current source resistor plus 0.2V for current source transistor C-E saturation). This will decrease further as the batteries wear out. Therefore sensors should be made to work with voltages between about 7V and 9.2V. The bottom right diode prevents overvoltage at the ADC input, pulling it down to 0.8V when the current source is on.