One-Shot Timer

DESCRIPTION

Some of my rechargeable tools do not like to be overcharged.  For example I have a rechargeable flashlight that needs a 15 hour charge and can be damaged if left on the charger for longer periods.  My cordless drill needs a 4 hour charge and the instructions say not to leave the battery pack in the charger for long periods.

In the past I used a lamp/appliance timer, like the ones for turning your lamps on and off when you are on vacation.  I would set the timer for the proper charge time, and the timer would turn the charger off after the time expired.  But then I would usually forget about it, and so the timer would run through a new charge cycle every day until I remembered to unplug the dang thing (after cursing about my forgetfulness).

This one-shot timer uses one of those lamp timers, but it will cycle only once.  You are guaranteed only 1 charge cycle.  You can use the one-shot timer to charge battery packs, or to run any appliance for a specific duration.  The lamp timer that I used in the One-Shot Timer allows setting a time period anywhere from 1/2 hour to 23.5 hours, in 1/2 hour increments. 

CIRCUIT DESCRIPTION

Well, it's pretty simple.  A relay supplies AC power to the timer.  The relay coil is powered by the timer, so that when the timer times-out the coil is de-energized.  Once the coil is de-energized and kills power to the timer, no more cycling can occur.

Hmm, lets see... the timer gets its power from the relay, and the relay coil gets its power from the timer.  To get the circuit started, there is a momentary pushbutton across the relay contacts.  Pressing the button will momentarily energize the relay and supply power to the timer.  Once timing begins, the relay will stay energized and the button can be released.  After the timer times-out the relay will de-energized and kill the circuit.

CONSTRUCTION

Nothing fancy here.  The only hard part is mounting the AC outlet into a case  The easiest way is to cut a large hole in the case and then use an electrical box cover.  The outlet will mount to the cover, then the cover will mount onto the case.  I got a little fancier with my case, I actually cut holes into it and mounted the outlet directly onto the case. 

Use a cheap AC extension cord to do some of the wiring.  Plug the cord into the outlet on the timer, then run the cord to the relay.  Cut the cord and connect the wires to the relay coil.  The other end of the extension cord can be used for plugging in your appliance.  Just wire it to the relay coil as well.

You will also need to connect an AC plug to your project, so that you can plug it into an outlet.  Connect the neutral wire from the AC plug directly to the AC outlet that is mounted on the case.  The hot wire from the AC plug should go to one side of the relay contact.  The other side of the relay contact should go to the AC outlet on the case.  You only need one contact on the relay.  If your relay has multiple contacts, just pick one to use.

For proper safety, you will want to keep track of the neutral and hot wires to make sure they are connected to the proper terminals throughout the circuit.  If you look on the end of the AC extension cord, you will notice that the blades are not both the same size.  One blade is wider than the other.  The narrow blade is the hot wire and the wide blade is neutral.  After cutting the extension cord, use an ohm meter to see which wire is going to the narrow blade.  This is the hot hire.  The other wire is neutral.  (check the end of this document for additional information)

Here are a couple pictures:

Instead of mounting the outlet directly to the box like I did, you can use an electrical box cover like this one:

After wiring everything up, use some 100% silicone sealer to glue the relay inside the box.  Don't bother getting a socket for the relay, just solder directly to the tabs on the relay.

If you use a metal box then it would be a good idea to use a 3-wire plug for power.  That way you can ground the box for safety.

OPERATION

SCHEMATIC

PARTS LIST 

Umm, neutral and hot what??

The wiring in your home typically consists of 3 conductors:  hot, neutral, and ground.

The HOT wire is the dangerous one, it always carries voltage.  If you measure from hot to ground, you will read 115VAC.

The NEUTRAL wire typically has no voltage.  In fact, if you look in your fuse box you will see that all the neutral wires are connected to ground (see below for a picture of the fuse box in my house).  So if you measure between neutral and ground, you should get close to 0V.  However the neutral IS still dangerous.  When an appliance is running, current will be flowing in the neutral wire.  Also, if there is some kind of malfunction with an appliance or there is some kind of wiring problem in your home (a loose connection somewhere, for example) it is possible to get 115VAC on the neutral wire.  So normally the neutral wire is 0V, but it can still be dangerous.

Which wire would you rather touch, hot or neutral?  Neutral, of course.  Modern outlets and modern extension cords are POLARIZED so that the plug will fit only one way (by having one of the blades wider than the other).  When wired properly the narrow blade is always HOT and the wide blade is always NEUTRAL.  This way appliance manufacturers will know which wire is hot, and they can design the appliance so that the hot wire is well insulated from the user.

If you look at the terminals of a modern outlet, one side will have silver screws and the other side will have brass screws (gold colored).  The brass screws are for the hot wire and the silver screws are for the neutral wire. 

So when wiring your one-shot timer, make sure you keep track of the hot and neutral connections.

If you have some old extension cords that are not polarized, toss them in the trash.  If you have old outlets in your home that are not polarized, it would be a good idea to replace them.

Here's the fuse box in my house:

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