jbn@glacier.STANFORD.EDU (John B. Nagle) (08/04/88)
I have an application that requires that I take a standard signal from an R/C receiver intended for a single servo and generate an on/off signal with it. The servo signal is the usual 22ms square wave with an on time of from 1ms to 2ms. We would consider > 1.5ms to be "on". Is there a standard way to do this? Certainly one could do it with a 555 timer and some standard TTL, but is there a solution with fewer parts? Please, no schemes involving servomotors driving mechanical switches. John Nagle
spcecdt@ucscb.UCSC.EDU (Space Cadet) (08/04/88)
In article <17606@glacier.STANFORD.EDU> jbn@glacier.STANFORD.EDU (John B. Nagle) writes: > > I have an application that requires that I take a standard signal >from an R/C receiver intended for a single servo and generate an on/off >signal with it. The servo signal is the usual 22ms square wave with an >on time of from 1ms to 2ms. We would consider > 1.5ms to be "on". >Is there a standard way to do this? Certainly one could do it with >a 555 timer and some standard TTL, but is there a solution with fewer >parts? Please, no schemes involving servomotors driving mechanical switches. > > John Nagle You can make two such circuits with a single CD4584 (CMOS hex Schmitt inverter). I've done it to implement the full-throttle bypass and brake circuits of a PWM for an RC car. Send the signal to the first inverter to get a nice rail-to-rail output (if the inverter is operating on a lower voltage supply than the receiver use a voltage divider on the input). Connect the inverter output to the input of another one through an RC circuit (inv output -> R -> C -> ground; R-C connection to next inverter input). The resistor & capacitor should be selected so that the second inverter will be triggered 1.5 ms after the first inverter changes state (with a 4584, a 0.01 uF cap and 250k resistor should be about right; you might want to use a pot instead). The output of the second inverter should be connected to the input of the third through a similar RC circuit, but with a switching diode (a 1N414 works well) in parallel with the resistor. If the input to the first inverter is a positive pulse, the diode should be connected with anode to the second inverter output and cathode to the capacitor and third inverter input. The RC should have a value that will keep the third inverter on for the ~20 ms between pulses (i.e, it should take more than 20 ms for the voltage on the capacitor to rise through the inverter's hysteresis band). A 3.3 M resistor and 0.01 uF cap should work. The diode will allow the brief low pulses from the second inverter to discharge the capacitor; the resistor will take > 20 ms to charge it so you will get a continuous high output from the third inverter when the input pulse is > 20 ms. Of course if you are going to drive much of anything you will need a transistor and maybe a relay on the output... The circuit has the advantage of extreme simplicity, low power (almost nothing), and relative insensitivity to power supply voltage as long as it doesn't change too fast.