Instant on - delay off circuit

[solvejskovlund]

I'd guess this kind of circuit is widely used. Is there an IC available that has this functionality?

I need a circuit that has a power supply (12V or 24V), an input signal and an output signal.
The wanted behavior is:
1) output goes ON immediately when power is connected (independent of input signal).
2) if input is ON, output should be ON
3) if input goes OFF for more than X seconds (adjustable 1-15), output should go OFF.
4) if input goes back ON before the output has gone off, the OFF-timer should be canceled.
5) if power goes off, the output should go off immediately. (no capacitor discharging through output)

I thought this sounded much like the interior light in a car, hence I'd guess there are IC's available with this functionality?

 

[DaveE]

Honestly there are probably a million solutions to this problem. Ranging from relay logic (i.e. no silicon) to uP. A classic would be something with a 555 timer (maybe one with counters built in for longer delays), but this is mostly because everyone knows them. My choice would be a MOSFET or SSR controlled by a tiny uC like a PIC. Then you could program it to do nearly any sequence. Often a large part of the design of circuits like this is the worst case analysis to make sure it does this simple task reliably for all conditions.

Yes, I'm sure you're right, there probably is an IC that does exactly what you want. I'm too lazy to search; you can start with TI.com and their timer or PMIC stuff. Analog, Infineon, Microchip are also likely manufacturers.

PS: I'm a big fan of stable oscillators with counters and logic to do timing over analog (capacitor charging) circuits like 555's or one-shots. The digital versions are simply more reliable, less glitchy; also easier to re-configure.

 

[anorlunda]

I thought this sounded much like the interior light in a car, hence I'd guess there are IC's available with this functionality?

But in a car, they can add that function to the car's computer. No additional special hardware required, just a bit of extra software.

 

[DaveE]

But in a car, they can add that function to the car's computer. No additional special hardware required, just a bit of extra software.

Lots of different uPs in a modern car. Many years ago I worked with an embedded systems guy that designed the controls on car seats. They wanted, and were required, to be completely isolated from the rest of the car, except getting a tiny bit of information. Same with radios, engine controls, instruments, etc. That's why there's a CAN bus in every car now. It's also why companies like Textron can sell (almost) the same stuff to several different OEMs. Too much integration is a business problem for many different reasons.

 

[DaveE]

Also, a PIC10 uC can be bought in a SOT23-6 package for less than $1. A 555 is bigger (SO-8, typically) and maybe 1/2 the cost, but then you have to add more capacitors and resistors to the circuit. Tiny cheap uCs have rightly replaced many historically analog functions. Often (not always) smaller cheaper, and better.

 

[berkeman]

I need a circuit that has a power supply (12V or 24V), an input signal and an output signal.

hence I'd guess there are IC's available with this functionality?

Yes, it's called a microcontroller (uC), which is similar to a microprocessor (uP), but with IO pins. The smaller ones are very inexpensive (less than $1, some much less than $1), and you can get little evaluation/widget boards for them that make it easy to program them and use them.

What does your output have to drive? What is the source of the input signal? If both input and output can be 5V or 3.3V referenced, you can just use a small Arduino demo board to implement this...

https://www.adafruit.com/product/2377#description

The Arduino Pro Mini is SparkFun's minimal design approach to Arduino. It's essentially a 3.3v Arduino that runs an 8 MHz bootloader on a super small, super thin board. There are no connectors attached - allowing you to add whatever connectors or wire you want without too much trouble.

The goal of the Pro Mini isn't to replace the Arduino so much as bring the cost down for more advanced users. The Arduino Uno is still the inimitable board but if you're comfortable working within voltage limitations, don't mind off-board USB, and want to customize your connectors, the Arduino Pro Mini's a great budget purchase. You will need an FTDI friend to program your Pro Mini, and we also recommend some plain header to solder it for breadboard use

Note that the FTDI port on the Pro Mini 3.3V expects both 3.3V power and data/logic! You will need to adapt the FTDI programming interface to make sure it is both 3V logic and power

Check out SparkFun's tutorial for more info on how to get started with the Arduino Pro Mini!

If you would like an Arduino IDE compatible board with USB, also check out the Pro Trinket 3.3V!

What is your level of experience working with circuits and perhaps writing simple programs for small uCs?

 

[Tom.G]

IF

( the electrical environment is harsh​

OR the temperature is/can be extreme​

OR your design time is not available )​

AND​

( there will be only one [or a few] unit needed )​

THEN

consider a Time Delay Relay (available as either electronic or electro-mechanical)​

ELSE

have fun with the design and programming!​

Cheers,
Tom

 

[solvejskovlund]

Thanks for lots of ideas.

uC's has become smaller than I thought. I had no idea they could be found smaller than a 555. That probably makes it consume less power than a 555 too? I need to look into these!

I did some arduino some years ago, but don't feel like they are suitable for this. (Adjusting voltage down/up will be more complex than building the circuit with a 555.)

I agree that modern cars probably uses software for this operation. But in the 1980's - 90's there should be some cars with that implemented in hardware.

Thanks for the idea of Time Delay Relay. I found an overview of variants, where one called
"OFF DELAY, Delay on Release, Delay on Break, Delay on De-Energization" seems much like what I'm looking for, except for the output on when power is connected.
https://www.macromatic.com/support/knowledge-base-articles/understanding-time-delay-relay-functions