timer switch

[normad]

im trying to design a timer circuit which would control 3 outputs as in -

t1 - t2 -- motor one runs
t2 - t3 -- motor two runs
t3 - t4 -- motor one runs backwards

and the time intervals should be variable within like 1 to 30 seconds.

according to my knowledge one way of doing this would be to use a pulse generators and three binary counters (please correct me if im wrong) but in this project we are supposed to stick to analog electronics. therefore my idea is to use something like an astable multivibrator. but that only provides a 2 way switch. so is there any way of achieving a three way switch using similar electronics?

 

[crutschow]

You could use three 555 timers. The timeout of the first (t2) would trigger the second, and the timeout of the second (t3) would trigger the third. The details are left as a exercise for the reader.

 

[normad]

thanks ill try to figure it out.. is it possible to change the timing on the spot? like by using a variable resistor? cause thats also a requirement..
ill try it out and be back with all the noobish questions

 

[crutschow]

The timing on a 555 can be readily adjusted using a potentiometer for the resistor that controls the period. Just Google "555 timer" for loads of info on how to use the chip.

 

[MrDEB]

**broken link removed**
this site will lead you in the right direction

 

[normad]

ok i looked up some articles and i think i understand the 555's monostable.. but when i tried simulating it on lt spice i fail to get the expected results..

i have a few questions..

so if the 555 triggers when the trigger voltage reaches 33% of vcc would it trigger if i connect the trigger to ground? else how am i supposed to trigger it?

in the simulation the output = vcc from the beggining and it doesnt go to zero when threshold reaches 66% vcc and the capacitor doesnt start discharging..

please help..
ive attached the ltspice file

 

[normad]

i put this on a new thread cause this seems to have moved down the forum and so far i didnt get any replies..i hope its not against the forum rules

https://www.electro-tech-online.com...ems-and-ltspice-simulation.104897/#post856498

 

[mneary]

When I run the simulation in your other thread, there is nothing hooked to the trigger and threshold pins. That schematic looks almost the same as the one in this thread.

 

[normad]

threshold is connected to the capacitor same as the discharge pin.. i checked just now even.. it is connected.. and the trigger is one of my questions. how can i trigger it? do i connect it to ground? but the simulation shows that the circuit is triggered because the capacitor is being charged. that means discharge pin is disconnected. doesnt that mean the 555 is triggered? but however it doesnt start to discharge even after the threshold reaches 66% vcc

 

[mneary]

But your trigger pin isn't connected to anything. How will timing get started?

 

[normad]

no worries the other thread showed me how to trigger thanks

 

[mneary]

Where is other thread? Could you bring us up to date please?

 

[normad]

https://www.electro-tech-online.com...ems-and-ltspice-simulation.104897/#post856498

 

[mneary]

i see.. thanks.. now im trying to make the end of the first pulse of the 555 trigger another 555.. this works but the problem is when i connect the output of the first 555 directly to the trigger of the second 555 the second triggers before the first because the output of the first is zero in the beginning..

ill upload the asc.. the second 555s output has two pulses.. nyway to limit that only to the last?

The output of U2 will always be high when U1 is low. You have to put a capacitor between stages.

 

[normad]

i didnt try it out but even if you put a capacitor the trigger of two will still be low and it would turn on.. so how does that change anything?

what im trying to do here is what crutschow suggested earlier in this thread

<quote>
You could use three 555 timers. The timeout of the first (t2) would trigger the second, and the timeout of the second (t3) would trigger the third. The details are left as a exercise for the reader.
</quote>
but the problem is that because the output of the first is low before its being triggered the second is triggered before the timeout of the first..

please help
thanks

 

[crutschow]

As I noted in my other thread, you use a coupling capacitor between stages with a pull-up resistor to the V+ at the output of the capacitor (the trigger input to the next stage). See the circuit on the right at **broken link removed** for example, where the preceding 555 output substitutes for the push-button. You just need the time-constant of the RC to be less then the 555 period of the stage being driven. A 10kΩ pull-up with a 0.1µF coupling cap will have a time-constant of 1ms, which is well below your minimum time of 1s. Add a diode across the pull-up resistor (cathode to V+) as shown in the referenced circuit to shunt the positive portion of the pulse when the driving 555 output goes high at the start of its period.

 

[mneary]

Why didn't my reply show up? I posted an .asc file that simulates correctly, I'll post it again later if it's still missing.

 

[mneary]

oh, other thread....

 

[normad]

hehe sorry about the thread confusion.. ill stick to one thread for my new posts

thanks a lot for the simulation.. my circuit is now complete.. but i still cannot understand the capacitor between 555 output and trigger works. i checked the simulation and the voltage on the trigger node increases simultaneously when the voltage on the output_1 node increases.. i cant understand how this is possible for a capacitor.. can anyone please point me to any thread or article that explains this behavior of a capacitor.. id really really appreciate it thanks again

 

[crutschow]

Capacitors pass AC and block DC. Here the capacitor is acting as a differentiator. It passes the high frequency edge of the pulse, but blocks the DC. Thus at each transition of the pulse you will see a spike with an exponential decay time-constant of RC.