Reference for "one-shot" circuit with input longer than pulse

--- View in Courier:

+V>-----------+------+-------+--------+--------+ | |K | |8 | [100K] [1N4148] [100K] +---+---+ | __ | | | 2|_ Vcc |3 | IN>--[0.1µF]--+------+-------|---O|T OUT|----|-----+ | 6| | | | +----|TH | [0.1µF] +------+ | 7|_ 555| | |K | +---O|D | | [DIODE] [COIL] | | GND | | | | |+ +---+---+ | +------+ [10µF] |1 | | | | | | GND>-------------------------+--------+--------+-----+

-- JF

Oops...

I mostly cut and pasted the schematic and forgot to change the timing resistor and capacitor.

For a 555:

t = 1.1RC,

so if you wanted to use something like a nice polyester capacitor at around 0.22µF, the equation rearranges to:

t 0.25s R = ------ = --------------- = 1.033E6 ohms ~ 1 megohm 1.1C 1.1 * 2.2E-7F

And the circuit changes to:

.+V>-----------+------+-------+--------+--------+ . | |K | |8 | . [100K] [1N4148] [1M] +---+---+ | .__ | | | 2|_ Vcc |3 | .IN>--[0.1µF]--+------+-------|---O|T OUT|----|-----+ . | 6| | | | . +----|TH | [0.1µF] +------+ . | 7|_ 555| | |K | . +---O|D | | [DIODE] [COIL] . | | GND | | | | . | +---+---+ | +------+ . [0.22µF] |1 | | . | | | | .GND>-------------------------+--------+--------+-----+

BTW, a standard 555 can output something like 100mA, so if your relay coil needs more than that you'll have to use a bipolar or MOSFET driver.

Thank you. One question, the "O"s at 2-TRIGGER and 7-DISCHARGE, do they mean to invert the input? YR

They indicate that the pin itself is "active low". Pin 2 (input) needs to be pulled low to trigger, while pin 7 (output) is held low to discharge the capacitor.

The above circuit is a standard 555 monostable (one-shot) configuration, with the trigger fed via an RC high-pass filter so that it only triggers on a falling edge rather than by the input being held low. The diode prevents the rising edge from taking the trigger input too far above V+.

As the time constant of the input filter (0.1uF * 100K = 10ms) is substantially less than that of the timer (0.22uF * 1M = 220ms), the trigger input will have returned to V+ by the end of the timing interval.