Hi all

Long long time since I had to think of this, I need to drive a 5v DC relay on for around 30 secs, then off momentarily, then on for another 30 secs, just using a single momentary push button to start the sequence.

I want to use TTL high to drive the relay through a ULN2803A. That part is no problem, nor is using something like a 555 timer circuit to produce an on period of around 30 secs, (plus or minus 5secs, it's not critical) but am getting boggled working out how to get two 30 sec on periods with a short break (say 200 to 1000ms) between the on periods from one momentary pushbutton press.

I also need to use another pushbutton to only do one 30 sec (ish) on timing period for the same relay.

Any thoughts?

Thanks

Christopher

 

Hi Christopher,
I’m not sure why you want to use a ULN2803 but for something like this I’d recommend using a simple low pin count processor. For a quick solution maybe a Seeeduino XIAO because it’s not expensive and uses the same IDE as the Arduino. Output pins are limited to a low output current but easy to drive the base of a transistor (via a resistor to limit current) to turn on the relay connected between 5V and the collector. Remember the protection diode!
The beauty of a processor is that it’s simple to change your timings and add other features

 

Hi Jerry.
I'm using a 2803 because I have several relays on a board being triggered by TTL non processor circuits via the 2803 and have a spare one to use. This is really a basic requirement, I will never need any more than a simple push button operation - at the moment I have to hold the button closed, hang around for 30 seconds, release and repress the button for another 30 secs - that's a whole minute wasted!! So a bit of TTL logic and a couple of 555 or even 7555 timers would do nicely - a processor, even a cheap one is a bit of overkill, aside from the fact that I haven't done any programming in nearly 50 years!
Regards, Christopher

 

555 or even 7555 timers would do nicely

This will give a single appx 30 second output using a LM555C.
On your first circuit is the 30 second timer to repeat continuously or for just one cycle?

 

Hi Sghioto this is looking promising
When the first 555 timer ends it should start another instance of the timer. When that second timer ends everything stops until the next start button press. If logic gating or latches can start a second instance of the first timer that would be an efficient use of hardware. The "off" period in between the two "on" periods can be very short, probably a couple of hundred milliseconds would do. (That's the bit I am having trouble with!).
Thanks
Christopher

 

A 555 Configured with maybe 80% duty cycle and a counter to resets it

 

Below is the LTspice simulation of a circuit that uses only one 555 timer to generate both pulses, but requires an added FF package to count the pulses, so not sure if that's any better than using two 555's.
(It does mean both 30s delays will be identical).
Your choice--

The particular FF in the circuit was selected for its negative edge clock trigger, which is needed for proper timing of this circuit.

 

Here's a crazy fully analog version comprising 3 timers using 2 LM555C and Q1.

 

Thank you both; I will play with these and report back!
Regards
Christopher

 

Hi Jerry.
I'm using a 2803 because I have several relays on a board being triggered by TTL non processor circuits via the 2803 and have a spare one to use. This is really a basic requirement, I will never need any more than a simple push button operation - at the moment I have to hold the button closed, hang around for 30 seconds, release and repress the button for another 30 secs - that's a whole minute wasted!! So a bit of TTL logic and a couple of 555 or even 7555 timers would do nicely - a processor, even a cheap one is a bit of overkill, aside from the fact that I haven't done any programming in nearly 50 years!
Regards, Christopher

Hi Christopher,
Thanks for taking the time to explain.
A single 555 monostable is usually triggered by a negative going pulse, so how about, when the first pulse returns to zero volts, can that trigger a second 555 monostable to produce a a second 30 second high output? By using the first monostable output going high, as well as turning on the relay, going through a diode to charge a capacitor which takes a brief period to drop voltage through a resistor when the 1st 555 output goes low, you'd get your brief delay before the 2nd monostable triggers. Need to get my head around whether it matters that the input to pin 2 of the 2nd monostable can stay low but I thin it can... Maybe use a 556 as it's a dual 555? Need to OR the 555 outputs.
Regards, Jerry

 

Here's another idea using a single 555.

EDIT: Removed three unnecessary diodes