555 Based Fast Recovery Turn-on Delay Timer

I searched everywhere for good ways of doing this but all of the ones I found had issues, so I designed my own.

This is a simple and easy-to-build but very effective on-delay timer with very fast reset/recovery, based on a 555 chip. As this is my first instructable, please bear with me if it's hard to follow. It might seem a bit technical for a complete novice, but ANYONE could build this. Just skip over the explanations and go straight to the parts and construction details!

The 555 timer is almost certainly the most versatile chip ever made, and there are thousands of existing explanations of its functions available on the web, so I'm not going to try to recreate them here. There are quite a few of these timer ideas on the net too, even one here on instructables, but it has one design issue (a missing freewheel diode) and no built-in voltage regulator. I also don't have the ability to design and make PCBs, and ordering one for a simple job like this seemed like MASSIVE overkill. So, stripboard it is!

One the key features of the 555 timer is its ability to operate at a wide range of voltages. This makes it great for use in automotive settings, because even though the car supply might vary or be noisy, it will operate happily at 9v which can be simply regulated down with a linear voltage regulator (here, a 78L09).

The very important feature I needed was the ability to restart the timer very quickly by removing the power and re-applying it. I've gone way past what I could practically have needed, because this is ready to go in less than 0.01s!

As a final note: What I actually built is slightly different to what's shown in the drawings. That's because I used up a very old small bit of strip board that was smaller than I technically should have used, and there wasn't enough space for the diode on the top of the board. Other than that, it's basically as the design shows. I've also used a greencap for one of the capacitors, as I could only find two .1uF ceramics among my components. I cheated on the regulator capacitors as the application example on the datasheet shows an input capacitor of 0.33uF, and I didn't have one. 0.1uF will be fine here as the load I'm drawing is very small.

I have used:

Semiconductors: 555 timer, 78L09 regulator.

Capacitors: 2 x 0.1uF, 1 x 0.33uF, 1 x 47uF, 1 x 10nF

Resistors: 1 x 10k, 1 x 6.8k, 1 x 10k pot (i used a 15 turn linear pot because I had one, but any trimpot will do)

Diode: 1 x 1n4004 (almost any diode will do as the relay coil is small)

Relay: DS2E-S-DC12V (because I had one - it's the 'sensitive' type and will switch at 7.5V)

PCB: A small piece of stripboard - you'll need 19 positions across 11 strips, at a minimum.

Tools: A soldering iron and solder, flush side cutters (to trim components), a cordless drill and a 3mm drill bit (to cut stripboard tracks).

I used www.falstad.com for circuit simulation, VeeCAD to design the stripboard, and KiCAD to prepare a netlist for VeeCAD to work from.

I tried to include the files from these, but instructables doesn't allow the file types ¯\_(ツ)_/¯

Look through the pics to help you understand some of this:

When using a 555, the usual setup is some variant of the conventional Vcc-R-R-C-G across the TRIGGER(2), THRESHOLD(6) and DISCHARGE(7) pins. Here I've taken a different approach.

To keep the maths simple I've simulated this at 12v, but the 555 doesn't care. I've got a fixed 5k resistance for the timing resistor here for simplicity, just because I wanted to get the mid-point of the 10k potentiometer adjustment to be near my target delay of 250ms. With the full 10k it understandably doubles the time to a bit over half a second. If you want more, increase either the timing resistor or the timing capacitor. Keeping the 10K resistor but moving to a 100uF capacitor gives about 1.1 seconds. All normal 555 timer calculators can give you this, with the "monostable" configuration.

https://ohmslawcalculator.com/555-monostable-calculator

I use a separate R/C/R trio to very briefly pull the TRIGGER (pin 2) low to be sure of setting the 555's output and starting the timer, and let it quickly rise above 1/3VCC to make sure it can't restart the timer after the first cycle. This takes about 0.7ms (shown in red). It only needs to be low long enough to be within the particular 555 variant's frequency response, and I don't take it any higher than I need to so that the recovery time is as fast as possible (that will make more sense later). The output can thus only possibly be low VERY briefly, but nowhere near long enough to operate the relay. A few hundred microamps flows through the relay for about 0.3 microseconds (shown in purple). Note that I could have used the RESET pin (4) to achieve the same result, but as far as I know the threshold voltages for that pin aren't necessarily the same for all variants of the 555, so I just tied it high instead. The TRIGGER voltage is an integral part of the design so it should be the same.

Once the timer is started, and the DISCHARGE pin (7) is at high impedance, the main timing capacitor starts to charge via the potentiometer (shown in green). As soon as it hits 2/3VCC the output goes low (in purple), which turns on the relay. It also starts discharging the relatively large timing capacitor (shown in pale blue), ready in advance for the next timing sequence. But, since TRIGGER is still above 1/3VCC the output does not go high again, and the relay stays on.

When power is removed (shown in orange), the capacitor on pin 2 rapidly discharges through the bypass resistor (in about 1ms, that TINY bit of red near the end, remember I kept the voltage as low as possible to keep this quick) and everything is ready to go again. The slowest thing in all of this is the relay!

Now just assemble it. I'm not super proud of my soldering here, but it works. I haven't unpacked my PCB holder yet!

The stripboard layout is the view from above, with X symbols marking the cuts. You'll see the small differences in my actual build, particularly the diode on the underside of the board. The 12V and GND connections are from the switched input supply, and the O-C and O-NO are the "Output Common" and "Output Normally Open" connections for the controlled device. I've made a few extra cuts just to keep internal circuitry tracks well separated from output soldering areas. For anyone looking carefully at the KiCAD drawing, yes one of the capacitors is labelled 0.1uF and another is 100nF - that's the same value of course, just written differently!

IMPORTANT NOTE 1: one of the links goes "through" (past, really) the leg of the CONTROL (pin 5)'s bypass capacitor, C5. Obviously this means it needs to be insulated. The others are all either clearly separate from other components, or run under them safely.

IMPORTANT NOTE 2: If using a small vertical trimpot, just don't solder the pin shown at the 'cut' position adjacent to the 555's pin 6. I used a horizontal 15 turn trimpot for accuracy and because I already had some, and you can see how its pins are arranged if you look closely. I had to make an extra track cut on my assembly, obviously.

This photo of the board was taken before the output cables were soldered into position.

You could get a lot fancier with the off-board connections and include screw terminals, but I didn't need them.

This will be sprayed with epoxy and housed in a small weatherproof box in the engine bay of my car with a couple of weatherproof connectors to plug it into the engine bay wiring. It will give me a very brief delay between my normal car horn sounding, and the obnoxiously loud Stebel Nautilus that screams "HEY LOOK OUT!". That way I can give a polite "bip bip" without people getting the wrong idea 😊😂

It's probably worth mentioning that this relay only switches the power to the main power relay that then drives the horn. There is no way this small signal relay could handle the current drawn by the horn!