555 in monostable mode triggered with NC switch?

[dog80]

Hello!

I want to make circuit with a 555 in monostable mode for a timed light switch, but for this specific application I have to use a normally closed switch for triggering. Is it possible?

[6502nop]

Yes.

Dave's hinted at discussing this very topic sometime soon.

See attached for the quick-and-dirty version...

nop

[dog80]

Thanks!  I tried it and it works

But what is the biggest resistor I could put there and still work? This is for a 9V battery operated device and even the tiniest current will drain the battery empty in a matter of days.

[6502nop]

"Trigger Current" is the datasheet value you're looking for. The trigger pin is just an input on one of the comparators, and is typically very high impedance. However, the value differs between manufacturers and even series (SE/NE/LM/etc.).

For the garden variety NE/LM type, the max trigger current is about 2uA. Using Ohm's law, R=E/I...
R = 9V / 0.000002A = 4500000 (4.5Meg)

It gets even crazier if you have the CMOS (TLC) version: Itrig=150pA!
R = 9V / 0.00000000015A = 60000000000 (60Giga-ohm)!

So, using a 500K will allow 18uA @ 9V, which should be small enough for most purposes, as the 555 itself will be drawing far more than that (~20-200mA typ.).

It would be very helpful to post your circuit so that we can make suggestions as to reducing current draw. As demonstrated, the trigger is the least of your problems if the output of the 555 is sinking 200mA when not timing. How the load is being used can drastically affect the battery life.

nop

[Zero999]

Thanks!  I tried it and it works

But what is the biggest resistor I could put there and still work? This is for a 9V battery operated device and even the tiniest current will drain the battery empty in a matter of days.

You need to use a CMOS TL555 or LMc7555. The NE555 has too higher current drain for continuous operation from a battery.

[dog80]

It would be very helpful to post your circuit so that we can make suggestions as to reducing current draw. As demonstrated, the trigger is the least of your problems if the output of the 555 is sinking 200mA when not timing. How the load is being used can drastically affect the battery life.

nop

My circuit is exactly like this here: http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/

I've used a 470K resistor and a 22uF cap. For the triggering I have used the circuit you posted earlier with a 330k resistor I believe. The output goes to a BD201 transistor that drives the LED.

The circuit draws 7mA when idling and 40mA when the LED is on. I removed the 555 from the socket and the circuit draws about 150uA, but the number falls after a while, so I guess this is some capacitor charging up.

[dog80]

This is for a light that will turn on whenever the front door is opened. I am using a door magnetic switch like those used in alarm systems. I've managed to find a NO switch but it is bulky and ugly, the nice looking switches are all NC.

[6502nop]

I'm with Hero999 on this one: Use the CMOS (TLC) version.

If your circuit can't be powered from a wall wart, then batteries are a given. The best option (for the enviornment and your wallet) is rechargables, and that means the most bang for the buck: LiPos.

Problem is, LiPos are a pain to recharge, and to use one with a standard 555 means getting a two-cell version. Unless you use the TLC555! It can be used with 3V cells - which is pretty much any LiPo. Since the advent of R/C cars and helicopters, small single-cell LiPos can be had for little money at most any hobby shop, and the chargers available are inexpensive and versatile (see Dave's video #397). If you already have these supplies, then just slap in a power connector to match your battery pack, and go!

Adafruit LiPo: http://adafruit.com/products/258
Adafruit Charger: http://adafruit.com/products/280

Now, you're not going to want to swap that battery out every day, or every other day, for that matter. So, low power draw is essential. Another CMOS advantage. I breadboarded the attached circuit, configured for a 33mA load (from your measurements), and played a bit with the values...

The pull-down resistor worked up to 10Mohm (the largest I had), so the trigger current isn't a problem, even at 3.3V (I was running from an 1117-33 regulator). Problem was, I was reading a draw of over 4mA. Until I realized I had hooked up my meter before the Vreg. Once corrected, the quiescent current dropped down to an expected 90uA level, jumping to about 35mA during the timing (LEDs on).

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Now, we can also reduce the current through the R1/C1 tank by making R1 as large as possible, and making C1 lower as a result. The problem with that is that even the TLC555 can only use caps down to a certain value before it hits the frequency limit (~300KHz for NE555, ~3MHz for TLC555). The other problem is that if you have a leaky cap, say 1uA or so, and you've limited the current to charge it by using a high value for R1, say 0.8uA, then the cap will never charge enough to trip, and it'll be forever timing.

Having said that, the circuit I breadboarded, using the values shown, gave a working timer with a delay of just over 30 seconds. Using the previous 1300mAH battery above, and figuring that we'll be recharging it at about the 50% level...
 .65AH / .00009AH = 7,222 (hours), or 300 days.
Of course, it'll be drawing 35-40mA every time the door is opened, and doing that for a minute every two openings (with my values), so...
 4mAminutes x 3 (six openings per day) = 12mAminutes / 60 = .0002AH per day.
 100 days = .02AH, 200 days = .04AH, and 300 days = .06AH, so:
 @300 days, we're at 650mAH - 60mAH = 590mAH.

Wow. The self-discharge rate of the LiPo should be higher than that, so I'd just recharge every 2-3 months just to be sure.

Also, you may not want to use that BD201. All transistors have a leakage value, and the power types tend to be leakier than the small-signal or general purpose types. Try your (or my) circuit again using a common 2N3904 or 2N2222. Since you're only drawing a 35-40mA load, darn near any transistor can handle it - no need for one of those big boys. It's probably where that extra 150uA went. Note also that the TLC555 can only source/sink about 3-4mA @ 3V, so adjust the value of the base resistor accordingly to provide enough bias for saturation for your load. A 1mA input will allow 100mA or so through a 2N3904. Plenty.

nop

Although not designed with the TLC version in mind, this 555 calculator doesn't mind shoving high values into it. The calc output isn't exactly spot-on, but it's close enough to get you going... http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/

[Online Simulations]

It is possible. One thing you must take care of is to keep triggering signal "high" until you really want to trigger - then you put it on "low" level for short period of time, while ne555 monostable circuit will hold your signal for given time. You can also simulate this on

http://www.cirvirlab.com/simulation/ne555_monostable_multivibrator_online.php

and read in more details on

http://www.cirvirlab.com/index.php/tutorials/97-ne555-monostable-multivibrator.html