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 2 msec 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 0.5 msec, the firmware also generates internal timer-overflow interrupts that determine the resolution of the simulated target's return echo. The interrupt-service routine increments the distance counter, and, when the counter's value equals the distance setting that R₁ supplies, output pin pA1 produces a 0.5-msec-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. Click here to download the firmware's assembler code.

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