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01-15-2017

Circuit forms fast, portable light pulser

NXP » ICM7555

SK Kaul and IK Kaul

EDN

The absence of a fast one-shot multivibrator in the entire TTL family, as well as the low-voltage swing and unwieldy supply requirements of ECL, drove us to exploit the fast transition times and low propagation delays of F-series gates. The application called for the implementation of a compact, portable, fast light pulser for field testing fast photomultipliers in gamma-ray astronomy work. The use of only two ICs helped to minimize the size and power consumption (Figure 1).

Circuit forms fast, portable light pulser
Figure 1. This circuit provides fast light pulses in a blue LED.

The normally high pulses at the output gate, G4, in IC2 have rise and fall times of approximately 2.5 nsec and a duration of less than 10 nsec, corresponding to three gate delays. These pulses are ideally suited to pull low the cathode of a fast, blue HLMP-CB15-type LED with the anode clamped at 5 V. The gate forces almost the entire 5 V supply voltage across the LED. This high swing ensures optimum brightness of the LED, which is soldered to the edge of a small pc-board strip. Rechargeable batteries are clamped onto the other side of the pc board. Using a CMOS version of the timer, IC1, the circuit has a current drain of less than 4 mA.

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