Hi,

I have an application, where external and internal power is available. I'd like the internal power to be supplied for, approximately, 1 minute after external power is removed. I have been trying to use a TLC555I combined with a relay, but the correct solution alludes me... Can someone please help?

Datasheet of TLC555I: https://www.ti.com/lit/ds/symlink/tlc555.pdf?ts=1726151450994

Circuit:

 

Is the timer circuit working? Perhaps a picture of your build would help.
Is the external power sharing ground with the internal?

 

Is the timer circuit working? Perhaps a picture of your build would help.
Is the external power sharing ground with the internal?

External power shares ground, yes. The output of the 555 chip never goes high, so no. Something is wrong there…

 

You might have a timer that is a little out of spec.

Well...maybe a lot out of spec.

 

External power shares ground, yes. The output of the 555 chip never goes high, so no. Something is wrong there…

TLC555 cannot drive a relay coil directly in that configuration. Use a transistor to activate the relay from pin3 output.

 

You might have a timer that is a little out of spec.

Well...maybe a lot out of spec.

Can you elaborate?

 

TLC555 cannot drive a relay coil directly in that configuration. Use a transistor to activate the relay from pin3 output.

So, if I remove the relay from the circuit I should get output from that schematic?

 

You should see almost 3.3 volts output on pin3 of the 555.

 

In your schematic, which net is internal power, which net is external power, and which connection is the control.

Also, please sketch a timing diagram to show which source is on when.

ak

 

TLC555 cannot drive a relay coil directly in that configuration. Use a transistor to activate the relay from pin3 output.

I missed that. 10mA max source. Relay needs 33mA.

 

I missed that. 10mA max source. Relay needs 33mA.

I missed that too... I have added a BC847 to the output to try to fix this...

However, doing some testing on the new curcuit design, the output stays high as long as V+ is present. Once V+ is removed it goes straight to +0.3V with no delay... Ideas?

 

In your schematic, which net is internal power, which net is external power, and which connection is the control.

Also, please sketch a timing diagram to show which source is on when.

ak

+3V3 is "internal" and V+ is "external".

Once V+ is powered, +3V3 is a result of V+ after the diode. Also powering +3V3, once the 555 has activated the relay, is two super-caps (C1 and C3). But I only want these to stay "on" for about a minute.

 

You should see almost 3.3 volts output on pin3 of the 555.

I can confirm that this is the case once the relay is removed. However behaviour still isn't correct... See #11.

 

I can confirm that this is the case once the relay is removed. However behaviour still isn't correct... See #11.

The V+ input to T1 must not be on longer than the duration of the timing cycle of R8 and C2.
Adding R2 and C4 is one way as shown below. This will provide a brief negative pulse to trigger the 555.