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

[[email protected]]

I need a "one-shot" circuit with enough power to close a small relay
for ~0.25s. The input trigger will go to ground and stay at ground forduration is << 0.25s. Can anyone provide a reference, on-line or
otherwise? Thank you, Yimin

 

[John Fields]

I need a "one-shot" circuit with enough power to close a small relay
for ~0.25s. The input trigger will go to ground and stay at ground for
duration is << 0.25s. Can anyone provide a reference, on-line or
otherwise? Thank you, Yimin

---
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>-------------------------+--------+--------+-----+

 

[John Fields]

I need a "one-shot" circuit with enough power to close a small relay
for ~0.25s. The input trigger will go to ground and stay at ground for
duration is << 0.25s. Can anyone provide a reference, on-line or
otherwise? Thank you, Yimin

---
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>-------------------------+--------+--------+-----+

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.

 

[[email protected]]

+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>-------------------------+--------+--------+-----+

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

 

[Nobody]

.+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.