Electronic Design
Configurable Logic Chip Stretches Pulses To Brighten LED Flash

Configurable Logic Chip Stretches Pulses To Brighten LED Flash

 

 

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A recent design project required a bright LED flash each time a 100-µs pulse occurred. The pulse repeated every 300 ms. Because the pulse was so short, driving the LED directly, even with a transistor driver, created a pulse too short to be seen well. So, I needed a “pulse stretcher” to increase the LED’s On period to about 1 ms.

In researching solutions, I found that the SN74LVC1G97 configurable multifunction logic gate from Texas Instruments could be wired in four ways to accommodate inputs and outputs of either polarity. The figure shows the four configurations and the Boolean logic equation for each.

Because the inputs are all of the Schmitt-Trigger type, the circuit can use slow rising (or falling) R-C inputs that allow output pulses up to several seconds in length. These circuits are all non-retriggerable. Any input pulse during the output active period will be ignored, though holding the input active longer than the output pulse width will keep the output active until the input pulse goes inactive.

The input pulses can be as short as 10 ns at supply voltages of 3 to 5 V. The output pulse width is approximately one time constant: T = R1 × C1. Due to manufacturing process variations, the Schmitt trigger levels may change the output timing slightly from device to device, but the timing is accurate enough for LED flashes, relay driving, etc.

Note the addition of Schottky diode D1 in the active-low circuits. Although the inputs are protected against negative-going spikes, the positive input spike that occurs when the output returns high can cause a damaging overvoltage condition on the input pin. D1 keeps this spike within the safe operating input maximum of 5.5 V for a 5-V power supply.

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