Help with designing a simple DC timing circuit?

[calkevin]

Hello, My son and I would like to build a one time night light with the following specifications:

1. Low voltage battery (like 3 volts) like 2 - AAA batteries (small to fit in a very small container)

2. Activated by a push-button switch
3. Once activated a single LED will light up
4. After 30 minutes? the light will turn off automatically to save battery life

Option:
A. Instead of a switch (item 2 above) provide a very small motion sensor to activate the light. If this device can not fit in the below space then omit this option.

The circuitry above needs to fit in a very small space like 3/4" x 3/4" by 5" long (not including the battery).

We appreciate any help or if there is a service that we can design this, we would be willing to pay for the simple design.

Thanks so much!

 

[DerStrom8]

Welcome to Electro-Tech!
It sounds to me like you want a 555 timer circuit wired in monostable (one-shot) mode. This will allow you to press a button once and the light will stay on for 30 minutes (can adjust resistor and capacitor values to keep it on for this amount of time). As for the motion sensor, I would normally suggest a simple light-sensitive switch (using a 393 comparator) to detect the shadow of a hand, but I'm assuming you'll be using this in the dark, where the circuit could easily trigger accidentally, and where shadows would be scarce. You may want to look into other motion sensor circuits (you can probably find some by googling "motion sensor schematic" or the like) to trigger the 555.
Let me know if you need help with the basic one-shot circuit with the 555.
Hope this helps, and good luck!
Der Strom

 

[MikeMl]

A standard 555 will not run on 3V, besides it will kill such a small battery very fast.

A CMOS 555 will come closer

 

[SABorn]

Yes a 555 would be a simple circuit and so would a 08M picaxe, as for a motion sensor i would think a mercury switch would do well for a tilt/invert sensor.

Another very simple circuit would be a mercury switch to charge a large capacitor (1000uf or the likes) and feed that to the base pin of a npn transistor switching the led on.

 

[MikeMl]

Another very simple circuit would be a mercury switch to charge a large capacitor (1000uf or the likes) and feed that to the base pin of a npn transistor switching the led on.

The base current of an NPN will discharge the capacitor much too fast. If the circuit could operate on a voltage higher than 3V, then the capacitor could be used to turn on the gate of an NFET, where the discharge time-constant is set with a 1meg or 10meg resistor.

 

[DerStrom8]

A standard 555 will not run on 3V, besides it will kill such a small battery very fast.

A CMOS 555 will come closer

Thanks, Mike. That's a good point. I guess I read through the post so fast that I missed the power specifications
A CMOS would work, but another idea would be to use a 9 volt battery (roughly the size of two AAAs). Even better would be a rechargeable one that you can recharge every couple of days. You may even be able to incorporate a "charging station" into the actual night light so that you can simply plug it it during the day and use it during the night.
Der Storm

 

[MikeMl]

Here is a fairly simple circuit. The NFet is sourced from Mouser. The switch can be a normally-open push button, tilt switch, or whatever.

 

[SABorn]

Ok there is more efficent ways of doing this, but a quick test on the bench with a few parts lying around gave 10+ minutes of good brightness of a white ultra bright led then leading to a steady fading of the led.

The switch can be replaced with a mercury tilt switch.

I didnt have a Fet handy so just used a transistor.
The capacitor can be doubled up to extend the time.
A 10 volt cap is not over large and would fit the size area ok.

Im sure someone will come up with a better solution but it dont get much simpler than this.

 

[MikeMl]

...

The switch can be replaced with a mercury tilt switch.

I didnt have a Fet handy so just used a transistor.
The capacitor can be doubled up to extend the time.
A 10 volt cap is not over large and would fit the size area ok.

...

The transistor is upside down. Putting in a second NPN connected as a Darlington with the one already there would make for longer delays with any given capacitor.

 

[calkevin]

Thanks guys! My response

So if I use a cMOS 555 and a 9 volt and pair up a few more capacitors, I'm happy with having the light stay on for 10 minutes. based on this the battery life should last fairly long since it's only used 10 minutes per night? Can someone show me a wiring diagram for this circuit? I'm a beginer and getting my son interested in electronics and also not scared off the dark......

I also heard something about using a binary counter? (in leu of a 555) supposedly it could have a longer delay time than a 555?

Anyway, you guys rock! and are very helpful and I appreciate all your time on this.

Kevin

 

[MikeMl]

Kevin, the problem of using a 555 is that it draws a lot of current (forever) after the timing cycle has elapsed. You have to go to substantial lengths to get the 555 to shut itself off to prevent discharging the battery during the 23.45 hours of the day when the circuit is inactive. A CMOS 555 has this problem, too, although at lower current draw compared to the bipolar 555.

The big adavantage of the two circuits made from discrete transistors already posted is that they shut themselves off after the timing cycle.

Here is yet another version. It uses more readily available PNP and NPN Darlington-connected transistors.

 

[kinarfi]

where in the world are you, one of us may live just around the corner from you

 

[calkevin]

I live in Northern California

 

[calkevin]

Mike,

Fantastic! Thanks so much. A few more questions:

1. On the legs of the transistors C, B and E were do they go in relation with your diagram?
2. What do you recomend for the voltage source for the night light, do you prefer a certain battery or quantity to get to the 3.2 volts? two AA's is only 3 volts?
3. Can I add a capacitor to the other one to get more time delay and if so would it be wired in parallel or sieries?


Thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[MikeMl]

...A few more questions:

1. On the legs of the transistors C, B and E were do they go in relation with your diagram?
2. What do you recomend for the voltage source for the night light, do you prefer a certain battery or quantity to get to the 3.2 volts? two AA's is only 3 volts?
3. Can I add a capacitor to the other one to get more time delay and if so would it be wired in parallel or sieries?

The lead with the arrow is Emitter; the lead at right angles to the bar is Base; the unadorned lead is the Collector.
A fresh Alkaline AA or AAA is 1.6V
The length of the timing period is proportional to the capacitor. If you want more time; get a bigger capacitor, or hook two in parallel, observing polarity. YOu should be able to buy 470uF, 1000uF, 1500uF or 2200uF.

 

[SABorn]

I corrected the transistor that was backwards in my earlier schematic (whoops) and added a resistor between the switch and cap as with a small switch the inrush current to large caps would be huge and burn the switch contacts, also added EBC labels to the transistor.

Yes i agree a second transistor would extend the period but did not add it to the schematic as that has been covered by others. (i actually expected bad press from such a simple crappy circuit)

Pete.