555 monostable push button delay off circuit with inverted output.

Hi everyone, I am trying to use the 555 timer to create a circuit that has a push button , that when pressed it sends a low output for 5 seconds then goes back to h igh. Also, if it is possible I would like the button to be a timer reset a s well so that if pressed again during the 5 seconds it adds more time othe rwise it can be pressed after time has ended for another 5 seconds. I need to keep the circuit as small as possible that way I can fit it into the ho using for it's application. I am going to use this to change the way a button on a existing circuit boa rd works. The existing button currently works by having current to it all the time then when it detects grounding(low) by button press, that circuit is complete and sends button pressed signal. I want to use the 555 timer circuit to make it so that when the button is p ressed it acts like it is being held down for 5 seconds every time without actually holding down the button like I have to currently.

Reply to
uriasjames
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on, that when pressed it sends a low output for 5 seconds then goes back to high. Also, if it is possible I would like the button to be a timer reset as well so that if pressed again during the 5 seconds it adds more time ot herwise it can be pressed after time has ended for another 5 seconds. I ne ed to keep the circuit as small as possible that way I can fit it into the housing for it's application.

oard works. The existing button currently works by having current to it al l the time then when it detects grounding(low) by button press, that circui t is complete and sends button pressed signal.

pressed it acts like it is being held down for 5 seconds every time withou t actually holding down the button like I have to currently.

That's called a re-triggerable monostable. To get an inverted (low going ) output, you trigger the THRESHOLD input with a logic high voltage. Let us k now what you find.

Reply to
bloggs.fredbloggs.fred

.-------------------------------------------------------------. | This is an ascii schematic, if the diagram appears garbled | | try switching to a fixed-pitch font (courier works well) | | pasting it into notepad works well on ms-windows. | | in google groups selecting 'fixed font' (top right of page) | | or view source (options) work too. | `-------------------------------------------------------------'

330 +--/\/\----+---------+------> | R2 | | new | | | button | | . . . .|. . . . | O | . VCC(8) . |-| | . . | O +---RES(4) OUT(3)-- | . 555 . +-----+--------TH(6) DIS(7)--------------+---> | | . . | existing | +--------TR(2) CV(5)----+ | button | | . . | O | \ | C1 . GND(1) . | |-| R1 / === . . . .|. . . . | C2 O | 470K \ | 10uF | === 100n | | | | | | +-----+--------------+----------+---------+---->

Out goes low when the button is pressed and will stay stay low until about 5 seconds after it is released, pressing again restarts the 5 seconds.

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Reply to
Jasen Betts

Thanks Jasen, I looked over your schematic and thanks for showing me how to look at it so it looks straight. I set up your circuit on a bread board. I do not have a 470k resister, so I had to use a 30k resister and a 100uf capacitor inste ad to get the 5 seconds like I wanted. I can stop at Fry's electronics to g et what I need to use the smaller capacitor to keep the circuit small once testing is complete. I used a LED and resistor in place of the existing bu tton to test it out. It worked great, even he re-trigger effect worked to add more time perfectly. So, I went ahead and changed out the LED and resister with the actual butto n on the device I am modifying and sadly it did not work. I am not sure wh y. If I had to guess, I would say it is because the ground is not actually a ground, maybe. The device seems to be very picky on what it uses for grou nd. I can touch the voltage wire from the device button directly to someth ing metal and it will activate a button press and I can, of course, use it' s own ground wire and touch them together and get a button press, but when I hook it to the breadboard circuit, I get nothing. Any ideas on why??

Reply to
uriasjames

I assumend for ground you meant the negative supply terminal.

How is the existing button connected inside the device? Does it have two terminals?

The 555 discharge output is an open collector pull-down of moderate strength. it's ok for running LEDs and relays, but not strong enough for larger loads.

If you connect a voltmeter across the existing button what voltage does it read? if you connect a ammeter what current?

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Reply to
Jasen Betts

Yes, it has two terminals inside the device. I am able to connect to one side directly to one terminal and touch the other to any negative terminal or metal on the circuit board and it registers a button press.

Ok, tested the voltage with a multi meter and it is at 1.5v. Also, when I test it, it registers as a button press. I assume it's because I am contacting both terminals.

Thanks again for your help.

Reply to
uriasjames

1.5V and registers as a press is a good sign. but also means that it should work with the 555.

Are you certain that the negative switch terminal is connected to the power supply ground? because the 555 circuit I drew needs it to be.

Also most 555s neew at-least 4.5V to run, is there enough voltage for the 555?

You can connect a LED and resistor from 555 pin 3 to pin 8 to monitor the operation of the timing circuit.

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Reply to
Jasen Betts

Damn, I wonder if that is the problem? I have been running the circuit on m y breadboard with 3.3v. It works with the LED but maybe not enough voltage for the device. Also, the device runs on two AA batteries, so that is two

1.5v. I am pretty sure that equals about 3.3v somehow. That is why I have been testing with 3.3v. I guess to verify if it will work I can change the voltage to 5v on the breadboard and hook it up to the device button and see if it works. I would like confirmation that it works and that is the issu e.

Isn't there a way to use a capacitor or something to step up the voltage fo r a short time to get it to work? Otherwise, I may have to abandon the who le project since I do not want to have a cord connected to the device and t here is no room for more batteries. It may also be another issue that the circuit is getting to big for the small space available in the housing. I am ok with making a hole and having the circuit attached to the outside of the housing if needed.

Reply to
uriasjames

Isn't there a way to use a capacitor or something to step up the voltage for a short time to get it to work? Otherwise, I may have to abandon the whole project since I do not want to have a cord connected to the device and there is no room for more batteries. It may also be another issue that the circuit is getting to big for the small space available in the housing. I am ok with making a hole and having the circuit attached to the outside of the housing if needed.

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Get a CMOS 555 such as the LMC555. That will work down to 1.5 volts.

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Reply to
tom

Put a 1 Farad cap across the battery. Instant short term power.

Reply to
Sjouke Burry

Aren't there some batteries out there with higher voltage, that come in the same basic form factor as two AA's?

Just a thought.

Reply to
mpm

there are low voltage 555s, you didn't say exactly which part you are using. eg: and LMC555 and 7555 will operate at 3V

possibly, I'm not sure how that would maintain the voltage if someone just holds the button.

OTOH you could do this.

+-[330]----> to +3.3 | | o |-| +---> to button active | o / | |/ +-----[1M]---| MPSA13 etc | |\| | -\ +---[1M]---+ | | | | +---||-----+----+---> to button ground timing will be less precise
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Reply to
Jasen Betts

I am currently using a NE555 timer. I am going to do what TOM suggested and try a LMC555. The link he left is for ebay but I like using Amazon so I a m going to see if I can find those on there. If not then I'll just buy the m from that link he left. Do I wire the circuit the same way as the first circuit design you suggested? If yes, then all I need to do it switch the NE555 for the LMC555 from the breadboard and test it.

Thanks again everyone, I will post if it works when I get them in.

Reply to
uriasjames

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