Circuit Diagram

This circuit really illustrates logic and timing control achieved without a microprocessor. This is useful for simple applications, as it removes a required-to-be-programmed component. It does require that you specify the task, comprehensively, before the design begins, as designing with logic is not versatile - making changes later is difficult. Changes are best done in software, on a microprocessor.

This application is fairly simple. There are two outputs required: Incline and Acceleration. These actually feed into an Engine Control Unit (ECU) which we also designed, on a hybrid electric vehicle. Pushing either "IncliStart" or "AccelStart" on this unit generates a test cycle into our ECU, while it is running on the vehicle, so that the vehicle performance can be monitored on a Dynamometer and compared with expectations. The rate and excursion (amplitude) of each test cycle is adjustable with potentiometers on this unit.

Pushing one of the push buttons (for instance AccelStart) brings the SET input of the RS latch IC1:C-IC1:D low, taking the output pin 10 high and starting a timing cycle. (Right-Click on the circuit diagram, and go "Open in New window" if you want to view the circuit alongside this discussion.) Capacitor C4 is charged though P3 with the RC time constant effectively setting the Accelerometer cycle time constant. Pot P4 divides the capacitor voltage to set the output amplitude, which is then buffered and multiplied by two by op-amp IC2:b. Depending on the state of bypass switch S3, the signal is either passed through to the output terminals, or the original ECU signals are passed through to the output terminals. Comparator IC3:D detects the top of the positive excursion of the test waveform and takes the RESET input of the RS latch IC1:C-IC1:D low, which starts the downward period of the test cycle. Comparator IC3:C detects when there is any test cycle in progress and lights the LED in the associated push button.

The power supply for most of the circuitry is an unusual voltage - 10V - which has been chosen as sufficiently lower than 12V to allow regulation even with a low battery, and sufficiently high enough to allow output signals that will fully test the range of the ECU inputs. This is one of the advantages of logic design - you do not have to constrain yourself to (or convert to and from) the microprocessor supply voltage. The power supply also includes a small amount of protection through a fusible resistor and zener on the input