Auto switch-off torch 2011-12-17

alec_t submitted a new article:

Auto Switch-Off Torch - An elderly acquaintance who couldn't cope with battery recharging was getting through dry cells rap

An elderly acquaintance who couldn't cope with battery recharging was getting through dry cells rapidly by leaving their torch switched on, so I built the following LED torch circuit which switches off automatically after about five minutes. Switching the torch off manually then resets the circuit.

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Operation

On closing the switch G1 output goes high and the capacitor charges slowly through R2. The high output of G3 provides base current for Tr1 via...

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This looks like a great idea with a number of possible applications.
Since the designer has actually incorporated his design in a (4.5 V) torch I guess it has a small "as built" footprint; so it would be nice to provide a link to a couple of pics or, devote a few lines here to how this was built.
Also :
1) I am not a complete novice at electronics but I have no real idea what the 4 "G" (gates) are?
My parts catalog does not list anything called a Gate.
2) 4.5 is not a common torch voltage, 3 & 6 being IMO much more common; so can this circuit be used on torches of this battery capacity, or is modification needed?

The article was not intended to be a complete constructional project guide; just a schematic. There is nothing complicated involved in building it. My circuit was built on an oddment of strip-board (Veroboard) and housed in a normal lantern-style torch.

I am not a complete novice at electronics but I have no real idea what the 4 "G" (gates) are?
My parts catalog does not list anything called a Gate.

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I'm surprised your catalog doesn't mention gates. Gates are standard logic circuits. The 4 gates G1-G4 shown are NAND gates contained in an IC type 4093, as identified on the schematic.

4.5 is not a common torch voltage, 3 & 6 being IMO much more common; so can this circuit be used on torches of this battery capacity, or is modification needed?

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A white LED needs at least 3V to drive it, so a 3V battery is unsuitable for this particular circuit (which requires a certain 'overhead' to allow the current to be controlled). 6V could be used but is wasteful. Why use 4 cells when 3 will do? If 6V is used the IC will need to be type CD4093B, not high speed CMOS type (which needs a 5V supply).

I tried this circuit but it happens that the led glows but does not turn off after 5 minutes..
can u please give me the exact transistor to be used...
please help me out..