What't the difference between a retriggerable one shot and a regular
relay? Isn't that the same thing?
--

Broderick Crawford

Highway Patrol

On Sat, 28 Jul 2007 21:08:09 -0400, Broderick Crawford wrote:

What't the difference between a retriggerable one shot and a regular
relay? Isn't that the same thing?

The basic one-shot will ignore a trigger pulse that arrives during its
'on' period. A retriggerable one-shot will stay 'on' for as long as
triggers arrive before the circuit reverts to the 'off' state. One of the
uses of the retriggerable version is as a watchdog timer, to raise an
alarm if an expected event doesn't happen on time.

On Jul 29, 7:26 pm, "Stephen J. Rush" wrote:
On Sat, 28 Jul 2007 21:08:09 -0400, Broderick Crawford wrote:
What't the difference between a retriggerable one shot and a regular
relay? Isn't that the same thing?

The basic one-shot will ignore a trigger pulse that arrives during its
'on' period. A retriggerable one-shot will stay 'on' for as long as
triggers arrive before the circuit reverts to the 'off' state. One of the
uses of the retriggerable version is as a watchdog timer, to raise an
alarm if an expected event doesn't happen on time.

Hi

This sounds like exactly the thing I need. I read up on relays on
internet but still I dont' find any that has an 'ON period'. Can you
explain more or tell me how to search for them.

What I want to do is as following. I want a small voltage (A) to
trigger an event (B). The B process must run for longer than the A
signal.

E.g you briefly press a button and a motor starts turning. Once a
complete revolution is done, it stops and you can press the button
again to start the process.

wrote:
On Jul 29, 7:26 pm, "Stephen J. Rush" wrote:

On Sat, 28 Jul 2007 21:08:09 -0400, Broderick Crawford wrote:

What't the difference between a retriggerable one shot and a regular
relay? Isn't that the same thing?

The basic one-shot will ignore a trigger pulse that arrives during its
'on' period. A retriggerable one-shot will stay 'on' for as long as
triggers arrive before the circuit reverts to the 'off' state. One of the
uses of the retriggerable version is as a watchdog timer, to raise an
alarm if an expected event doesn't happen on time.


Hi

This sounds like exactly the thing I need. I read up on relays on
internet but still I dont' find any that has an 'ON period'. Can you
explain more or tell me how to search for them.

What I want to do is as following. I want a small voltage (A) to
trigger an event (B). The B process must run for longer than the A
signal.

E.g you briefly press a button and a motor starts turning. Once a
complete revolution is done, it stops and you can press the button
again to start the process.

Couldnt you just use a regular relay, have it latch until a limit switch
is activated by the motor's revolution?
The limit switch is open while the motor is "at home position".
The button "shorts out" the limit switch, turning on the relay. The
limit closes when motor turns, holding relay on until limit opens again
at home position.
You might have to hold the button until the limit closes, but this could
be overcome by using another relay that "latches on".
I can draw you a simple diagram, if you are interested.
Of course, if the button and limit switch can handle the motor's
current, you probably wouldnt even need a relay.

Hi

Yes please draw away. I finally found the LATCH keyword on wikipedia
(should have started my search there).

Won't a normal latch relay to it. Input button to start it, and then
use the motor to physically trigger the reset when one revolution is
completed.

wrote:
Hi

Yes please draw away. I finally found the LATCH keyword on wikipedia
(should have started my search there).

Won't a normal latch relay to it. Input button to start it, and then
use the motor to physically trigger the reset when one revolution is
completed.

Yes, that should work too.
Are you going to use a limit switch that will activate at the end of the
revolution?
The only problem I could see with using a latch relay would be that you
would have to use a normally open limit, which is closed when motor
reaches home position.
The relay will have the reset coil energized by the limit switch when
the motor is at rest, at home.
This may prevent you from latching the relay with the switch.
I guess you just need to make sure the reset doesnt overide the set on
the relay.
Also make sure the reset coil is rated for continious duty, so it doesnt
get hot while motor is at home position.
There are several ways you can do this. If the motor is DC, you can use
a SCR to fire on until the limit switch opens the circuit. You may have
to use a resistor in parallel with the motor to keep the SCR from
turning off when the motor brushes break the current flow.
If the motor is AC, for simplicity purposes I would just use a relay.
Does the pushbutton really need to be a one shot?
If you hold the pushbutton, do you still want the motor to stop at home,
having to release the button and re-press it to start the motor again?
Or do you want the motor to run continiuosly while the button is pressed?
Is the control voltage AC or DC? Is it the same as the supply for the motor?
Does the motor "stop on a dime", or will it coast when the power is
turned off?
These are simple basics that you will need to know before designing any
kind of control for this.

Skenny wrote:
wrote:

Hi

Yes please draw away. I finally found the LATCH keyword on wikipedia
(should have started my search there).

Won't a normal latch relay to it. Input button to start it, and then
use the motor to physically trigger the reset when one revolution is
completed.

Yes, that should work too.
Are you going to use a limit switch that will activate at the end of the
revolution?
The only problem I could see with using a latch relay would be that you
would have to use a normally open limit, which is closed when motor
reaches home position.
The relay will have the reset coil energized by the limit switch when
the motor is at rest, at home.
This may prevent you from latching the relay with the switch.
I guess you just need to make sure the reset doesnt overide the set on
the relay.
Also make sure the reset coil is rated for continious duty, so it doesnt
get hot while motor is at home position.
There are several ways you can do this. If the motor is DC, you can use
a SCR to fire on until the limit switch opens the circuit. You may have
to use a resistor in parallel with the motor to keep the SCR from
turning off when the motor brushes break the current flow.
If the motor is AC, for simplicity purposes I would just use a relay.
Does the pushbutton really need to be a one shot?
If you hold the pushbutton, do you still want the motor to stop at home,
having to release the button and re-press it to start the motor again?
Or do you want the motor to run continiuosly while the button is pressed?
Is the control voltage AC or DC? Is it the same as the supply for the
motor?
Does the motor "stop on a dime", or will it coast when the power is
turned off?
These are simple basics that you will need to know before designing any
kind of control for this.

Here is a drawing of the first circuit, which assumes the control
voltage is same as motor voltage, and it uses a regular relay, which
should be cheaper than a latch relay.
Motor runs continuously if button is held.
http://relaycircuits.blogspot.com/