Momentary-activated timer with no idle power consumption

[w1zard]

I'm trying to work out a simple circuit which takes a momentary input from a switch, switches a relay on for about 40 seconds, then switches the entire circuit off.

The key is that the circuit is completely off during idle, so it doesn't run the battery down in extended idle periods.

The timing and momentary parts I think are best achieved using a 555, but the problem is triggering the circuit to stay on during the timing, and switching the entire circuit off again afterwards. Can anyone help?

Thank you!

 

[ericgibbs]

I'm trying to work out a simple circuit which takes a momentary input from a switch, switches a relay on for about 40 seconds, then switches the entire circuit off.

The key is that the circuit is completely off during idle, so it doesn't run the battery down in extended idle periods.

The timing and momentary parts I think are best achieved using a 555, but the problem is triggering the circuit to stay on during the timing, and switching the entire circuit off again afterwards. Can anyone help?

Thank you!

hi,
Whats the specification of the relay, what voltage is it switching and what do you plan to use as a battery.?

 

[w1zard]

hi,
Whats the specification of the relay, what voltage is it switching and what do you plan to use as a battery.?

The relay can be anything it needs to be (I can source it if necessary) - ideally a 6V or 12V coil as I've got these lying around. It will be switching a 12v/2A inductive load, and powered by a 12v sealed lead-acid battery (around 1.5Ah). The circuit will share the same power source as the load, even if I have to step it down for the circuit.

Thanks

 

[KMoffett]

This should work. 555 power-on rest.

Ken

 

[MikeMl]

Another way to do it. Zero current draw after the timing event. R2 limits charging current. Works with any 12V relay. NFet not critical; almost anything you have in your junk box will work...

 

[w1zard]

Thank you both for the circuits - much appreciated

I've prototyped the first (555) circuit on breadboard, and while it works, it acts a bit unpredictably. When the switch is shorted, it almost always takes at least two shorts before the relay engages. Holding it shorted for longer makes no difference - usually it takes two brief shorts before it starts the timer. It seems to work better if the shorts are closer together - i.e. if I short once, then wait 10 seconds, short again - it generally doesn't work. Two brief shorts close together almost always works.

Do you have any idea what is going on here? I've probed around the circuit and triple-checked it, but can't see anything obvious. Just to check - the unpolarised caps are 100nF ceramics aren't they?

The relay has a 100 milliohm coil and is 12v, so should be fine, the supply is 12v regulated, and all other components are to spec (the ceramics are marked 104 - I think this is correct).

Thanks for the help

 

[KMoffett]

The relay has a 100 milliohm coil and is 12v,

0.1 ohms is not a logical value for the coil resistance. Did you mean 100 milliamp coil current?

Ken

 

[w1zard]

Hi Ken
Sorry, I copied that from the data sheet without running it past my brain! 100milliohm was the contact resistance.

It looks as though the coil resistance is 720 Ohms from the data sheet - this seems quite high to me, as it's a sub-miniature relay. The data sheet PDF can be found here (click PDF link next to 'Manufacturer Data Sheet'):

**broken link removed**

I could be reading it wrong, but for model LU-12 (12V), it seems to suggest 720 Ohms.

Any advice on getting the circuit going would be appreciated

 

[KMoffett]

I breadboarded my circuit. What you see is probably due to contact bounce in either the relay contacts or the pushbutton. To lengthen the power-up trigger time, change the R3 to 100KΩ. This works flawlessly for me with a 12VDC/720Ω relay.

That said, Mike's circuit is very simple. I like it! You should give that a shot too.

 

[w1zard]

That's great - really appreciate the help. Changing R3 to 100K fixed the 'bounce' problem on the switch/relay that was there before

Just to complicate the matter, now we have this bit working, I was also hoping to utilise the momentary switch as a 'reset to idle' switch as well - i.e. if pushed when the timer is running, it resets it and sends it back to idle, without starting the next countup. I was thinking in terms of some kind of flip-flop in front of the timing circuit - but again this must have no draw in idle. Do you think this is possible?

I will certainly try Mike's circuit - it looks very simple - I just didn't have anything anywhere near a 3.3meg resistor (I've ordered some), or the exact 5.6uF capacitor (had close, but not identical - again, some are on the way).

I'm also interested in the simulation Mike did - is this LTSpice? I need to get into simulation, but there's so many available I don't know where to invest my time learning - is this a good one to start with? Would it be capable of simulating something complex like your 555 circuit?

 

[KMoffett]

Mike's 5.6uF cap is probably just a number picked for the simulation. That's not a standard value. That particular RC combination works in simulation, but may not work in the real world. Electrolytic capacitors have a wide tolerance range that can vary as much +/- 20% from the labeled value. You can parallel capacitors and string resistors in series to get what you need for a particular time.

I'll have to think about the reset feature. Easy to do with a second NC (normally-closed) pushbutton. Just put in series with the +12V supply. Pushing it cuts the power to the timer.

Ken

 

[w1zard]

Okay, I'll bear that in mind when the other components arrive for Mike's circuit. I knew the capacitor was non-standard, but managed to find them on Ebay for virtually nothing so I'll give them a go.

Yes, the NC reset switch would be the easy option wouldn't it! Unfortunately I'm constrained on panel space, so a dual-purpose button is the only way I can go. I thought it might be a challenge to get working!

 

[KMoffett]

Would you accept a 3-position toggle switch: momentary-ON/center-OFF/momentary-ON? SET by pushing it one way and RESET by pushing it the other way. This would require the addition of a second relay.

Ken

 

[w1zard]

It's going to be operated by the elderly, so the push button is likely to be easier to press for them than a toggle switch - which is the reason why I'm looking at the momentary switch in the first place. I don't think they will have the dexterity to flick a toggle switch, unfortunately

 

[KMoffett]

It's going to be operated by the elderly, so the push button is likely to be easier to press for them than a toggle switch - which is the reason why I'm looking at the momentary switch in the first place. I don't think they will have the dexterity to flick a toggle switch, unfortunately

w1zard,

Does this have "Life Safety" implications? Can you explain the whole project?

It may be easier to help solve your problem, than help solve your solution.

Ken

 

[w1zard]

Sure - it's an automatic plant watering system which will be operated by an elderly person who can no longer do it manually. I have built and have working the timing and pump system, which is in place and up and running. This uses the trigger coming from a salvaged board of an LCD programmable mains timer, adapted to run off a 12v battery. This drives a relay which pumps the water through a standard irrigation system.

While it works well, and provides great flexibility due to the 7-day 24h programmable timer, the minimum 'on' time is 1 minute - that is it will switch on at 6:00 and then off again at 6:01. This is a limitation of the timer used. So the first part of the solution is to put this 555 circuit in front of the programmable timer - allowing a customisable 'on' time - about 40-50 seconds seems to be the correct amount of 'watering'.

The second issue is that the plants might need 'topping up', so the system needs a manual trigger so they can be watered for, say, 10 seconds, then switched off. This is where the trigger/reset button comes in.

The space is limited in this system as it is IP68 protected due to the hostile (wet!) environment, and the current enclosure already houses the timer board, 12v lead-acid battery, a solar panel, etc. Space on the panel is therefore limited.

It will also be much easier for the elderly person to operate if there is just one button that can be hit to switch it manually on, then off again - anything else would likely be difficult to accurately press.

The 'reset' also serves as a 'get out' if the timer happens to trigger while someone is in there - there's a few seconds after hearing the pump start before you get wet

With regard to Life Safety - as this is low voltage, low current and fused throughout, I can't see it being a danger to anyone in its current form. Other than getting them wet.

Hope this makes it clearer

 

[KMoffett]

OK, at this point I would just program an 8-pin microcontroller. I did a circuit/program that would do either 'push-on/program-off' or 'push-on/push-off' with one button. Timing could be a fixed program value, or a variable time set by an external potentiometer.

Ken

 

[w1zard]

Yep I was starting to think that way too! I've never used microcontrollers before, and have none of the hardware, so I'll need to decide whether to go down the PIC or AVR route, whether to build a programmer or buy one, etc. But it would give me far more flexibility and I have a background in software engineering so this bit shouldn't be too much of an issue.

I imagine I have a long journey ahead!

 

[MikeMl]

5.6uF is a standard value. The time delay is about 2.5RC = 2.8 * 3.3e6*5.6e-6 = 45sec. You can use 1meg and 15uF. Note that a snubber diode is not needed because of the slow turn off of the FET. A second push button to discharge the timing capacitor would shutoff the relay early.

 

[w1zard]

5.6uF is a standard value. The time delay is about 2.5RC = 2.8 * 3.3e6*5.6e-6 = 45sec. You can use 1meg and 15uF. Note that a snubber diode is not needed because of the slow turn off of the FET. A second push button to discharge the timing capacitor would shutoff the relay early.

Hi Mike
Thanks for the info - I've picked up some 5.6uF caps - they were a bit rare, but they exist! They're just not a rating generally stocked in the sites I buy from, but I'm sure they're standard

I also had to buy the FET (couldn't find one lying around), so will give your revised circuit a go when it arrives.

Was it LTSpice that you used for the simulation? Is this the best software for me to learn as a beginner?