HELP: Design of a timing system

glenjoy · Oct 7, 2006

timing system push button

I would like to ask if you have an idea on how to create a timing system for this kind of circuit arrangement. The timing system/circuit will be inside the said blackbox.

Note: Do not suggest to re-arrange the circuit. The circuit arrangement or the position of the button/switch will always be that way next to the motor.

The design requirement will be like this, everytime the user press the button/switch, the motor will turn for a certain period of time, say 1 minute, as programmed in the timing circuit inside the blackbox.

Maybe you are thinking to suggest a 555 monostable circuit, yes it will do, and it is the one required here.

The question is, how to place the 555 monostable circuit in the design for it to cut the power after the desired time of operation.

Btw, the circuit must do the timed operation everytime the user presses the button.

Thanks.

VVV · Oct 7, 2006

If you can use a relay, then use the oldest trick in the book: have a relay contact short the switch. The 555 is what keeps the relay going, so when it times out the relay disengages and cuts the power.
Pushing the button again makes the cycle repeat itself.

glenjoy · Oct 7, 2006

VVV said:
If you can use a relay, then use the oldest trick in the book: have a relay contact short the switch. The 555 is what keeps the relay going, so when it times out the relay disengages and cuts the power.
Pushing the button again makes the cycle repeat itself.

Can you elaborate more, btw, as I said in my post, you cannot modify anything after the box.

pete · Oct 7, 2006

What type of switch is that? Toggle switch? Latching switch? Or push to "ON". Elaborate first the detail of your switch.

newelltech · Oct 8, 2006

The only way this would work, looking at the schematic is with a momentary push button. And so I asssume that this is the way it will work:
1. The user pushes and holds the button.
2. The motor runs for 1 minute and shuts itself off.
3. The user lets go of the button.
4. Cycle repeats

Here's a solution for you. I'll let you work out the details. All the parts are inside the box.
1. In series with the push button, add a small resistor and the normally closed contacts of a relay.
2. Add a large resistor (maybe 470K) across the relay contacts.
3. Add a pnp transistor with collector to +9, base to the small resistor and collector to another resistor to ground. The resistors should be calculated so that with the motor on, the transistor turns on and saturates pulling the collector resistor to about 8.5 V. Add a resistor in series with the base if you need to protect it from over current.
4. Trigger a 555 from the collector resistor so that when the motor turns on the 555 is triggered and produces a single 1 minute pulse.
5. Add a flip-flop to control the relay. Set it when the 555 is done and reset it when the voltage to the push button drops to 0.

How it works, starts with relay off and contacts closed.
1. User pushes and holds the button
2. Motor turns on and the transistor turns on.
3. Collector voltage rises and triggers the 555
4. 555 times for 1 minute
5. After 1 minute the 555 times out and goes low
6. Flip-flop is set when timer goes low.
7 Flip-flop set energizes the relay and n.c. contacts open up turning off the motor.
8. Pushbutton voltage is still low while user holds button.
9. User lets go of button, pushbutton voltage rises to 9v and flip-flop is reset.
10 Flip-flop reset de-energizes relay and contacts close ready for next cycle.

Notes:
Make sure flip-flop is reset on power-up.
An easy way to trigger flip-flops is to ac couple the pulses so it triggers on the appropriate edge.
You could use a transistor in place of the relay - I just used a relay so it would be easier to describe.

I'll let you draw the schematic and pick the components.

glenjoy · Oct 8, 2006

pete said:
What type of switch is that? Toggle switch? Latching switch? Or push to "ON". Elaborate first the detail of your switch.

Btw, the circuit must do the timed operation everytime the user presses the button.

This statement clearly tells that it does not matter be it momentary or not, btw check the schematic first and READ.

Added after 3 minutes:

newelltech said:
The only way this would work, looking at the schematic is with a momentary push button. And so I asssume that this is the way it will work:
1. The user pushes and holds the button.
2. The motor runs for 1 minute and shuts itself off.
3. The user lets go of the button.
4. Cycle repeats

Here's a solution for you. I'll let you work out the details. All the parts are inside the box.
1. In series with the push button, add a small resistor and the normally closed contacts of a relay.
2. Add a large resistor (maybe 470K) across the relay contacts.
3. Add a pnp transistor with collector to +9, base to the small resistor and collector to another resistor to ground. The resistors should be calculated so that with the motor on, the transistor turns on and saturates pulling the collector resistor to about 8.5 V. Add a resistor in series with the base if you need to protect it from over current.
4. Trigger a 555 from the collector resistor so that when the motor turns on the 555 is triggered and produces a single 1 minute pulse.
5. Add a flip-flop to control the relay. Set it when the 555 is done and reset it when the voltage to the push button drops to 0.

How it works, starts with relay off and contacts closed.
1. User pushes and holds the button
2. Motor turns on and the transistor turns on.
3. Collector voltage rises and triggers the 555
4. 555 times for 1 minute
5. After 1 minute the 555 times out and goes low
6. Flip-flop is set when timer goes low.
7 Flip-flop set energizes the relay and n.c. contacts open up turning off the motor.
8. Pushbutton voltage is still low while user holds button.
9. User lets go of button, pushbutton voltage rises to 9v and flip-flop is reset.
10 Flip-flop reset de-energizes relay and contacts close ready for next cycle.

Notes:
Make sure flip-flop is reset on power-up.
An easy way to trigger flip-flops is to ac couple the pulses so it triggers on the appropriate edge.
You could use a transistor in place of the relay - I just used a relay so it would be easier to describe.

I'll let you draw the schematic and pick the components.

We cannot add a series resistor in the button, it will create too much heat. The motor draws about 1A to 2A ( I will check this later ). What we can add in series are series and/or power fets.

Thanks.

VVV · Oct 8, 2006

After a better understanding of the requirements I was going to suggest exaclty what newelltech has already suggested: a pnp to detect the motor current to know when to start up and a large resistor across the relay contact to detect an open switch.

I do not understand why you say the power would be too large. The power dissipation is only going to be in the low ohm resistor used to detect the motor current. You need roughly a 1 ohm resistor for 1A, which would dissipate 1W.
But if that is a concern, then you can use an opamp to amplify the signal, which allows a much smaller sense resistor and so waste much less power in that resistor.

glenjoy · Oct 8, 2006

VVV said:
After a better understanding of the requirements I was going to suggest exaclty what newelltech has already suggested: a pnp to detect the motor current to know when to start up and a large resistor across the relay contact to detect an open switch.

I do not understand why you say the power would be too large. The power dissipation is only going to be in the low ohm resistor used to detect the motor current. You need roughly a 1 ohm resistor for 1A, which would dissipate 1W.
But if that is a concern, then you can use an opamp to amplify the signal, which allows a much smaller sense resistor and so waste much less power in that resistor.

Can we start now the block diagram or the schematic?

pete · Oct 9, 2006

Mr. Glenjoy wrote:
"The design requirement will be like this, everytime the user press the button/switch, the motor will turn for a certain period of time, say 1 minute, as programmed in the timing circuit inside the blackbox."

Based on your schematic, seems that your switch is a push to "ON". If you release the switch before 1 min lapse, surely the motor will turn off. Your 1 min program will be useless. Kindly clarify your assumptions and avoid sarcastic answers as you have been doing before in this forum. Regards from your best friend - Cris

glenjoy · Oct 9, 2006

pete said:
Mr. Glenjoy wrote:
"The design requirement will be like this, everytime the user press the button/switch, the motor will turn for a certain period of time, say 1 minute, as programmed in the timing circuit inside the blackbox."

Based on your schematic, seems that your switch is a push to "ON". If you release the switch before 1 min lapse, surely the motor will turn off. Your 1 min program will be useless. Kindly clarify your assumptions and avoid sarcastic answers as you have been doing before in this forum. Regards from your best friend - Cris

1 minute is an example, turning off/releasing the switch before the desired time elapses must turn the motor off, that is a desired operation, so there is nothing useless in that.

Oh I see, regards from a lady in DIGITEL also. And from your class beadles. From the guys also in capitol.

pete · Oct 10, 2006

Thank you for your clarifications. Will see and others if we can help you Mr. Glenjoy.
Will ask from the ECE group of Dr. Regondola, your previous ECE students and your previous companies who have lot experts from there. You have a good question though, let me think of it. Regards...

glenjoy · Oct 10, 2006

pete said:
Thank you for your clarifications. Will see and others if we can help you Mr. Glenjoy.
Will ask from the ECE group of Dr. Regondola, your previous ECE students and your previous companies who have lot experts from there. You have a good question though, let me think of it. Regards...

The so called raging hormones?

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HELP: Design of a timing system

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