In normal suburban driving you pass through so many different speed
zones that it can be a nuisance having to switch speed settings. The
speed display can also be a distraction. This circuit eliminates the
display and the need for speed selection. Each time you exceed a
particular speed setting (eg, 40km/h, 50km/h, etc), a piezo buzzer will
beep. Speed pulses are fed to the base of Q1 and the resulting waveform
at its collector is fed via an RC network to the input of an LM2917
frequency-to-voltage converter, IC1. The resulting voltage is fed to
three comparators (IC2d-IC2b) which have the reference voltages at their
inverting inputs set by 10-turn trimpots VR1, VR2 & VR3. The output
of each comparator is applied via another RC network to the gate of an
SCR. The anodes of the three SCRs are commoned connected to the
inverting input of the remaining comparator, IC2a.
Its non-inverting input is set to +2.3V by trimpot VR4. In use, once you
exceed the speed setting for a particular comparator, its associated
SCR briefly conducts to pull pin 2 of IC2a low and a short beep is
emitted by the piezo buzzer. Then, as you exceed the next speed setting,
another beep will be heard. The idea is make each speed setting a few
km/h higher than actual so that if you are driving at the correct speed
in a given zone, the buzzer will not sound. But as you increase speed,
the buzzer will beep once as you exceed the speed setting for each
zone. In this way, there is no need to continually switch speed
settings as you drive through different zones and you can choose to
ignore beeps that are not "illegal"
.Electronics Circuit Application
Author: Col Edwards - Copyright: Silicon Chip Electronics Magazine
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