Hacking a 5" B/W Television

Mikerocontroller

· I tried the digital potentiometer experiments in the What's a Microcontroller? kit and I was looking
for an application for the digital pot.·· I took apart a Colby 5" B/W portable TV and found that the
TV tuner uses a 100K pot.· I was thinking of converting the TV to digital tuning with preset station
recall and IR remote control.· I don't know the wattage rating of the pot in question.· I checked
different sites for an 8-Pin DIP 100K digital potentiometer with UP/DOWN interface and I found that the current ratings are very low for these chips- most are less than 1mA.· Does anyone here know
if a digital pot would work in this application?·
·

LoopyByteloose

Well you have hit upon the one great weakness of digital pots. They are extremely low amperage devices.

The most general indicators of a pot's power requirements are its actual size and what what it is connected to. I suppose that you could stage the digital pot to control more than 1ma with a 2n2222 transistor [noparse][[/noparse]100ma maybe] or an op amp [noparse][[/noparse]25ma or so, but more predictable]. With either device, one doesn't actually have to use a matching digital pot, but the output must be over the same effective range as the original pot.

I suspect the extremes of the 100k pots range are not used and that it goes to a VCO chip [noparse][[/noparse]voltage controlled oscillator]. One milliamp just might work in this context.

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Phil Pilgrim (PhiPi)

Do the math. To get 1 mA to flow through a 100K resistor, you'd have to apply 100V — an unlikely condition in a transistorized TV.

If the pot is being used as a divider with the wiper being the output to a high-impedance load (most likely scenario), you should have no problem. OTOH, if a voltage is applied to the wiper or the wiper is driving a low-impedance load, you will have to compute the worst-case current based on the wiper's extreme settings.

One thing to consider with a digital pot is its resolution. With 256 different positions, for example, will you be guaranteed that each station you want to view will correspond exactly to one of them? I have my doubts. Another thing to consider is the voltage range for the pot terminals. This is typically limited to Vss-to-Vdd. So you will want to make sure the device you pick can handle the voltages present in the TV.

-Phil

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