Momentary Circuits

[stf5060]

I need to make 3 circuits, all of which have pretty much the same purpose. Basically I need them to be activated for a short time interval (varying seconds for each circuit) and then stop and not turn back on until they are given power again. One of the circuits will control a 12V motor, and the other 2 are going to be controlling 24V solenoid valves. First of all, would a 555 timer circuit be the easiest/best way to do it? This is a circuit diagram some one gave me that they said I should use. Is there any changes I need for this? And How would I go about varying the amount of time the motor/valves are given power for? Thanks for any help you can give me!

 

[KMoffett]

So did you not try my circuit (bad design from late night)...with StgWookie's modifications? https://forum.allaboutcircuits.com/showthread.php?t=60292

Ken

 

[stf5060]

I posted a few questions I had but could not get any answers to them. Could you please take a look?

 

[KMoffett]

The changes SgtWookie stated were correct. Though what I posted would work, it was late, I was tired, and the his changes would be the standard configuration.
In the left schematic delete R1 and D1. Change R3 to 100K.
In the right schematic delete D1. Swap the positions of R1 and D2. Change R1 to 1K. Change R3 to 100K.

Ken

 

[stf5060]

ok, now what is the max power rating that I need the parts to withstand? Because I was under the impression it had be around 8 Watts, but will less do? And which of the resistors/capacitors do i change the values for to change the time using the calculator from the L555 link you provided? Thanks a bunch

 

[KMoffett]

All capacitors must be rated >12VDC. R1 in the 24v circuit is 1/2W and the other resistors can be 1/8W or 1/4W. R3 can be any wattage pot.

Ken

 

[stf5060]

Great thanks for the help Ken. Can I still use this **broken link removed** to calculate the timing for it?

 

[KMoffett]

Yes. Pick a standard value for C1 in that calculator, and select different values for R1. I used a pot so I could easily set the time in the real circuit.

Ken

 

[stf5060]

Sounds great, I really appreciate the help. I'm gonna start ordering the parts and then get to building. What is the significance of the 1-15 Sec Pulse Width that's written near the R3 on the circuit.

 

[KMoffett]

That is the variable ON-time range with the timing components shown.

Ken

 

[alec_t]

Depending on the wall-warts used to provide the 12V and 24V supplies, I think you may find that at power-up the TRIG input reaches the trigger level before the 555 chip supply rises to its proper level, so the timer won't start. IMHO the time constant on the TRIG input is far too small for reliable triggering. I'd increase it to ~200ms at least; e.g. make R2 220k and make C1 1uF.

 

[KMoffett]

Your opinion need not be humble alec. A good option.

Ken

 

[stf5060]

my wal warts are 12V 100mA and 24V 300mA. So it'd be a good idea to make R2 220k and C1 0.1 uF. I was reading that it is a good idea to keep C1 high and R1 lower to compensate for leakage

 

[KMoffett]

R2=220K and C1=1.0uF as alec said.

Ken

 

[stf5060]

But that means for it to run for 1 second I need a 1,000,000 Kohm resistor....

 

[KMoffett]

R2 and C1 are there to create a brief trigger signal to start the timing. R3/R4 and C2 are the 1-5 second components.

Ken

 

[stf5060]

So i use C2, and which R should I use for the calculator on the other site?

 

[KMoffett]

C2 in my circuit is C1 in the calculator. R3+R4 in my circuit is R1 in the calculator. R4 sets the shortest time period. R3(max)+R4 sets the longest time. If you are not going make your circuits adjustable, delete R3 and select a value for R4 that gives you the time you want.

Ken

 

[stf5060]

Is there any significance to the value 47 uF? With a capacitor value that small I still need 20,000 Kohms to get a 1 second time period. Can I simply increase the C2 to like 1000 uF?

 

[KMoffett]

So what is the problem do you envision with 47uF and 20K for the timing.

Ken