555 and relay

[swilsonNC]

Can you please look at my circuit and tell me if I should be able to run a regular automotive relay from pin 3 of second timer? I have tried it on a breadboard and I can't get it to cycle the relay. It will cycle the relay off of the 1st timer if I try that. Possible losing too much current or voltage. How much voltage will I lose through this circuit do you think?

Should I be using a different relay for this instead?

Last question, How can I make this work exactly the same using a 556 instead of a 555. The single Vcc confuses me if I try to wire it like the 2 555's.

Thanks

View attachment 63066

 

[alec_t]

The CV pin normally connects to ground via a cap. Why do you have it connected to OUT? That may be affecting things.

 

[swilsonNC]

Thats a good dang question and I see what you are saying. I just didn't catch that. I am new to this stuff so I guess you learn as you go. Thats why I come on here and ask the experts advice.

Now speaking of the 556 timer. Is there anything special I need to do to that schematic to make the 556 work instead other than moving CV?

 

[alec_t]

Should I be using a different relay for this instead?

No relay shown! What coil resistance and voltage rating does your relay have? The OUT pin of a 555 (or a 556) can handle 200mA, which should be plenty for most relay coils.

How can I make this work exactly the same using a 556 instead of a 555. The single Vcc confuses me if I try to wire it like the 2 555's.

The two timer circuits inside the 556 share a single Vcc pin and share a single ground pin. Simple.

BTW, both 555s are shown configured as monostables. Is that what you intend? You talk of 'cycling' the relay. If you want it to operate repeatedly then you should configure the 555 driving it as an astable.

 

[swilsonNC]

The relay I have is a standard 30a automotive relay. The coil draws 160mA at 12v.

Ok looking at the schematic I have I see the 556 layout now.

I got the schematic from **broken link removed**, its the 2 stage time delay circuit. I am worried about the standard voltage lose per each 555 (1.7v I think). Would the 556 have 2 x 1.7v loss also. The supply voltage will be 14v or so and the relay is rated 12v.

Sorry, by cycling I meant on and then off after 2 secs and that's it.

 

[alec_t]

I am worried about the standard voltage lose per each 555 (1.7v I think)

There isn't a standard voltage loss. The output voltage of a 555 will be close to the supply voltage if it is acting as a source for small currents. For large currents the voltage will fall by an amount dependent on current (at 200mA the rated fall is 2.5V if the supply is 15V). A 555 is rated for 200mA output current so your relay will be drawing close to that. IMO it would be better either to use a relay needing less current or to use the 555 to drive a transistor buffer and use the buffer to drive the relay.

 

[swilsonNC]

Ok, gotcha on the voltages. I put the 556 together in LTSpice and on a breadboard and get the same results with both. My 2nd half of the 556 will output as soon as you apply voltage to the circuit. It should only output after the output of the 1st half triggers it. It works correctly using 2 555's but for some reason the single 556 triggering the 2nd half as soon as voltage is applied. I am guessing its something to do with the common ground in the chip. Is there a way to fix this so it acts like the 555's? When voltage is applied Output 1 should go high for 5 or 6 secs. When it goes low is when the other half is triggered to go high for 1 sec.

View attachment 63103
View attachment 63104

Here are the 555 timers and how it is suppose to work.

View attachment 63105
View attachment 63106

Just so you know normally on the 555's and 556 Out1 goes high for 5 sec's when voltage is applied and when it goes low the 2nd timer is triggered. I just didn't show the output of first timer in traces.

 

[alec_t]

When I run the LTS sim of your 2 x 555 circuit, Output2 goes high as soon as power is applied.
Try connecting Reset2 to +V via 100k and to ground via 10nF. That ensures the second 555 is held in the reset state at power-up.

 

[swilsonNC]

Thats weird. The 2 x 555 sim in my LTS the output2 stays low until output 1 goes low. output1 does go high as soon as power is applied.

Anyhow I did some fiddling with the 556 circuit and I think I got it working. Only changed to discharge2 and threshold2 pins. I tested via led and via diodes + relay and it works a treat.

Here is what I came up with:

View attachment 63112

And trace is Green is supply voltage, blue is output1, and red is output2

View attachment 63113

BTW, the relay is pulling 113mA so it should be fine I am guessing. What do you think?

 

[ericgibbs]

hi sw,
I assume you have set the internal Vdd for the the 556 to 14V,?

If not , right click the 556 symbol and change the Vdd from 9V [default] to 14V.

E.

 

[alec_t]

BTW, the relay is pulling 113mA so it should be fine I am guessing. What do you think?

Yup, that's fine. I trust the relay coil has the usual spike-suppression diode across it.

 

[swilsonNC]

Yup got the diodes in their proper place.

I got the design down now and it seems to work pretty good on breadboard except for one little problem.

When I initially apply voltage sometimes the output thats driving the relay goes high for a second and then resumes as normal. Every so often the timing seems off and it triggers a fuzz early. The early trigger isn't that big of a deal but the voltage at initial power is.

View attachment 63208
View attachment 63209

I am not using ground of R1 and R2 as the switch to trigger it. I am using the +V to R1 and R2 to trigger it. In other words R1 and R2 are connected to a relay that exists already in original system. When original system is operating normal no voltage goes to R1 and R2. When an interrupt is seen then R1 and R2 get voltage. Basically put a momentary push-button on R1 and R2 to test with. It should work if button is held down when main voltage is applied and it should work after main voltage is applied anytime you press the button. This will simulate what happens on the original system. I hope you follow me. Thanks

BTW I have tried CONT1 with .01u to ground but doesn't help.

 

[alec_t]

The early trigger isn't that big of a deal but the voltage at initial power is.

You haven't connected the reset pins as required.
If you want the timer to stop and not complete if the 14V is missing then you need to take the reset pins low when the 14V is missing.
Your 'fuzzy' timing is probably due to the absence of a 10n cap on CONT1. If that pin picks up noise (which it will if there are straggling wires on a breadboard) it will vary the timing.

Try the attached.

 

[swilsonNC]

Alec_t,
Thanks for the help. I tried your schematic and it works great in LTS but on the breadboard it doesn't work correctly. Most of the time it doesn't trigger at all. I should be able to trigger it over and over by keeping the voltage on V2 but it won't. Seems like after the first trigger it is done. Not sure what is going on there. I have checked and rechecked my breadboard connections and all is good. BTW I tried with 2 555's and 1 556 and same thing.

 

[alec_t]

I should be able to trigger it over and over by keeping the voltage on V2 but it won't. Seems like after the first trigger it is done.

I queried that in post #4 but in post #5 you said you didn't want it to trigger repeatedly

 

[swilsonNC]

Oh yes, so sorry alec_t. The trigger will be either on or off. Mostly off but it might need to trigger and run the timers all over again at any time while V1 is present. At the moment I can cycle V1 on your schematic and it works but I need to be able to trigger as many times as I want via V2. Again output Delay 5secs and on 1sec. Also if V2 goes to 0v before 5 sec delay is up timers should stop until next 14v at v2.

 

[swilsonNC]

Ok, so here I will try to sum up what happens.

V1 = Constant 14V from switch on dash.
V2 = signal from ECM on vehicle (not directly from ECM) I just know that when everything is OK and the system is ready you don't get any voltage. When system is not ok or ready you get 14V.
Out2 = relay that opens and closes a circuit that restarts the whole "system". It also restarts V2 in a sense that you get a dead circuit until the relay closes the circuit back.

I need a circuit that will stay powered via V1 (which won't go to 0V unless switch is flipped off) but trigger off of V2's voltage. This way the relay can open for a full 1 sec instead of just on and instant back off. I don't think a relay blinking on and off would work too good in the long run. I want the timers to get constant voltage so they aren't hammered on and off either.

1. I flip the switch on the dash on then V1 gets 14V constant.
2. At the same time my V2 source has 14V and a timer on the truck counts to 3 secs.
3. If ECM says everything is ok then after the 3 secs I get 0V at V2 and system comes up as normal. If ECM says everything is not OK then I keep 14V at V2 and the system needs resetting to try again.
4. If it goes past 3 secs with 14V then 2 more secs later (5 secs for 555 timer) the Out2 will energize the relay for 1 sec and opens the reset circuit.
5. When the relay on Out2 turns off it closes the circuit which it opened and the system starts over at #2 above.

I need this to kick in when V2 has 14V at any time while the switch is on.

 

[audioguru]

Did anybody mention that the high voltage produced by the inductance of the relay coil when it turns off will destroy the 555 unless the TWO diodes are added as shown on some datasheets?
Then the 160mA relay coil gets only about 3V less than the supply voltage.

 

[swilsonNC]

Yes, as I said before the diodes are in their proper place. I understand I will get less voltage on the output that the relay is connected to. So far with the circuits I have tried at 12V V1 and V2 and with diodes in place the voltage and current will energize the relay no problem so I am assuming 14V will help it that much more.

 

[alec_t]

I think this is what you're after. Both timers will repeat while V2 is high and will stop as soon as V2 goes low. Unfortunately the first cycle of the 5 sec timer is more like 7.7 secs. Is that likely to be a problem?