Microprocessor, linear potentiometer deliver echo pulses

For designers of radar or sonar echo-ranging systems, an echo imitator can ease development and adjustment chores by generating a controllable pulse that’s similar to an incoming echo signal. A decade ago, you’d probably use several 555 timers and their associated RC circuits to design an echo imitator. As Figure 1 shows, today’s version uses only two components: linear potentiometer R1 and a small, low end microcontroller, IC1. An external trigger pulse applied to IC1’s pA0 input pin triggers the first of two pulses delivered to output pin pA1. For ultrasonic-receiver testing, the optional first pulse imitates 2msec of post-trigger sensor ringing that limits the minimum reception distance. You can also use this pulse to synchronize an oscilloscope.



Linear potentiometer R1, a Panasonic model EVA-JGTJ20B14, sets the echo pulse’s delay time. Microcontroller IC1, a Freescale MC68HC908QT2 8-bit Flash memory device, includes four 8-bit successive – approximation ADCs, one of which digitizes the voltage at R1’s sliding contact. For this application, the firmware divides the digitized potentiometer readings by four to match the system’s reception range. Every 0.5msec, the firmware alsogenerates internal timer-overflow interrupts that determine the resolution of the simulated target’s return echo. The interruptservice routine increments the distance counter, and, when the counter’s value equals thedistance setting that R1 supplies, output pin pA1 produces a 0.5msec wide echo pulse. Thus, the echo’s delay time tracks the potentiometer’s sliding-contact position. Although this Design Idea features a Freescale microcontroller, you can use others that include an ADC. To download the firmware’s assembler code, view the online version of this Design Idea at www.edn.com.