[10.2.09] Quick Modification to a Tap Light

There's a dark stair well i traverse every night to get to my apartment. I modified a cheap tap-light to be [Tap-> stay on for 40 seconds, then shut off]. The mod takes all of 10 minutes to do, and is really useful if you don't want to go back and shut it off.

And its 4 components.

This is basic electronics, no arduino/ microcontroller / 555.
(no standby power usage either)

This is also useful in like a closet, where you hit the button look for things, and it shuts off by itself.
Component Purpose Image
N channel Mosfet
For those who are just starting EE things, the mosfet is basically a switch.
image from:

Momentary Switch Replacement for the switch provided on the unit. Is connected when pushed and otherwise open circuit.
from http://alexplorer.net/guitar/mods/momentary.jpg
Capacitor Timing capacitor. Based on Capacitance & Resistance, you determine on time constant (or amount of time the system stays on)

Resistor (timing) Timing Resistor, Based on C&R you determine on time
9v battery & clip I ran it on a 9v, because i have a bunch of em

White Led's  way more energy efficient and brighter than the incandescent it comes with. (image from digikey)

Build &Test:
Action Purpose Image
Basic Schematic a momentary switch, mosfet, led's resistor and cap. Time constant approximated by how long it would take for the bleed resistor to reduce the starting voltage (9v) to the dropout voltage of the gate (depends on the mosfet). If your working with unlabeled parts, test values until you find the time constant you want, as gate capacitance is probably unknown.

Adding a momentary switch replace the existing pushbutton on/off switch (non momentary in my case)

momentary switches are on only while your holding them down, like when you smack the taplight
Soldering it together, boardless style Who says you need perfboard :)

Top View Here, you can see the momentary switch, and the newly installed led's Yes, there are only 2, they were scavenged from something. Yes, you can use more than 2.
Everything shoved into the  back of the tap-light I shoved everything into the back of the tap-light. The system could have used 4AA's like it was designed to, however i was in a rush and decided i'd rather run it on some 9v's.
Button it up Align the springs, check that the button fits properly and button the whole thing up.
Analog circuit design has been relatively abandoned in comparison to digital design. Sure, its more complex to get working, and sometimes finicky in terms of noise, but it excels in other areas, like filtering and other simpler designs. In the simple cases, like above, its wonderful. When i first contemplated this idea, i was thinking, 'well i would have to get a 555, or an attiny...', then i realized that was silly and a waste of good hardware. The general idea here is simplicity can also be elegant. Oh, you also get that nice analog fade as it shuts off.  :)

What is Actually Going On
1. Everything is in the off state:
The modified taplight is off, just sitting there minding its own business. No significant currents happen anywhere.
2. You Smack The Taplight
The voltage of the battery suddenly appears at the gate of the mosfet, turning on the led's. This also fills the Timing capacitor rather quickly with charge.
3. The you climb up the stairs, after releasing the tap light.

The charge that was stored in the cap slowly depletes across the timing resistor.
As the cap voltage drops, the mosfet remains on, until the voltage approaches the fet's threshold, wherein it begins to reduce the current flow (fet now operates in the analog region)
4. The Timing Cap is depleted

Capacitor becomes depleted, Mosfet turns off, led's turn off. System consumes virtually no power.

Here's a flash video if it working (while still apart) next to a stopwatch

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Other note:
This site is intended to explain things so others can get a grasp at electronics.
While this particular project may not be helpful to you, it might be helpful to someone younger / less experienced than you. (middle schoolers)

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Rensselaer Polytechnic Institute 
Electrical & Electrical Power