Need 555 timer to start when power is turned off, sort of...

555 simulator

Say i want to have a motor running, and i want to hook up a 555 timer so that when i turn off the motor switch, the motor continues to run for the time i have set on my 555 timer. I dunno how to hook this up, especially as i don't want to interupt the power to the motor, not even momentarily as i turn off the power switch, i'd want it to continue spinning at a constant rate until the timer runs out.

So the motor power switch provides instant power on, and an uniterrupted power supply until the timer runs out.

Thanks for all help guys

555 li on off

There are many ways to do this. One way is to use a DPDT switch. One section of the switch drives the control pin of a transistor (base for BJT, gate for MOSFET). This section is your normal on-off.

The other section of the switch drives the trigger input of the 555. The 555 is configured as a monostable multivibrator. Once triggered, it will produce an output pulse whose width is determined by you. The 555 output is used to drive the same control pin of the motor transistor as the previous switch section. This is called a Wired-OR connection. Either input can drive the control pin. Since the 555 is a totem pole output, you will have to include a blocking diode or at least a big resistor to keep the 555 from loading down the control pin.

You will have to experiment with some RC components on the control pin to keep the motor from seeing a glitch as it transitions from switch to 555 timer.

While using the 555 timer is a workable solution for a simple, one of a kind circuit, I would not recommend a production design using it. Microcontrollers are too cheap today. A small 8 pin microcontroller can use a cheaper SPST switch and fewer external parts to do the same job. You have some upfront time writing the microcode, but the payoff is well worth it.

555 timer motor gate

555s can be very fickle when used in timer mode, they are very prone to false triggering when near any source of interference. I think you will end up tearing your hair out when used in the presence of an electrically noisy motor. You can buy timed off relays with variable timers. Also as Banjo suggested, a cheap micro PIC16C628 etc may be a better option. Downside to this of course is unless you are already into micros, you will probably not have a programmer. However there are many free cut down compilers out there for C, Basic etc. I think CCS do a free limited code one for C, if you want to go in that direction. As to setting the time, you do not even need a micro with built in ADC. Just have a pin connected to a link as input, with another pin as output driving a led. Have a routine that counts up and flashes the led, say once a second. When you get to the required time, remove the link, and then have the routine write to flash. You can then read from this within your program, to get the time delay you set.
Good Luck

on-off 555

The easiest thing that comes to mind is this:
Use the output of the 555 to drive the motor, either with a driver or with a relay.

The 555 is configured as a monostable and the switch actually shorts to ground the TRIG input. Thus, when you want to turn on the motor, you short to gnd the TRIG input and the 555 will turn the motor on. Since the TRIG input is shorted to GND the 555 will not time out as long as the switch is on. When you open the switch that is when the 555 will start its timing and when it times out it stops your motor. There will not be any glitches with this approach.

how to keep a 555 timer from false triggering

Thanks guys This is just a one off project, and i've got no experience with microcontrollers.

I was actually hoping to do it the way you say VVV, but when i tried on my 555 simulator program, the 555 timer started as soon as i triggered the input, not when i let go!

I'll try the circuit on out my breadboard, hopefully you're right VVV

555 timer false triggering

I think you probably misread VVVs post. The timer is supposed to trigger right away and operate the motor relay. If you leave the trigger output connected to ground, it will never time out. Removing the input will then allow it to time out and switch off the motor after this period. I think this is what you want? I would suggest that you use the cmos version of 555 rather than the standard one, this is much less sensitive to spurious triggering. However the drive output is much lower, and you will need a transistor or similar to drive the relay. I gave up using 555s years ago, as they caused me too much grief. They are ok for astables or battery powered stuff, but need careful layout and suppression in noisy enviroments. It may work well for you, simple and chap. Have a go.

how to keep a 555 from triggering during power up

Hi mvagusta,

Sorry about the confusion. What I meant is in the attached schematic. Basically, in the astable schematic disconnect the resistor from the discharge and pull it to GND with your switch. The resistor can be eliminated, but it protects the contacts of the switch when the cap discharges, especially if the cap is large.
Make sure R2 is low enough such that when the switch is closed the voltage on the cap drops below the 1/3Vcc threshold. The no-CMOS version of the 555 requires a cap on the CONTROL pin.

Again, sorry about the error.
VVV