Electronic Circuits and Projects Forum

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

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.

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 http:// home.cogeco.ca/~rpaisley4/LM555.html, 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.

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.

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.

556bad.jpg
556badtrace.jpg

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

555good.jpg
555goodtrace.jpg

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.

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.

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.