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Using transistors to control an RF remote

I am trying to use some NPN transistors to control a Hampton Bay RF remote (for my ceiling fans). I have it all working, but my digitally controlled remote seems to have less range than my other unmodified remotes.

The buttons on the remote connect +9V to ground when pressed. So, I used an NPN transistor and hooked the emitter to ground, the collector to this +9V, and hooked the base to a 5V digital output.

When the button is pressed, I measured a current of 0.65mA. The power source is also 9V, so I determined the resistance to be 13846 Ohms (0.65mA and a 9V drop).

So, I did my calculations and decided to use a 1.5MOhm resistor between the +5V and the base. I came up with this value as follows:

Vi - 0.7 4.3V ib = -------- ==> 3.25nA = ---- ==> Rb ~= 1.5MOhms Rb Rb

I'm suspicious that somehow my transistor is limiting the range of the remote. Perhaps is it providing a restrictive amount of current flow?

I will appreciate any help with this. I can email you high-resolution pictures of the remote control's circuit boards. For reference, here is my circuit:

Vcc | | Rc | | / | _/ |__/ Vi ---- Rb ----| |---_ | \ \ | | Gnd

B = 200 Vcc = 9V (remote battery) Vi = 5V (digital output of Weeder board)

Reply to
Kirk Bauer
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As I understand it, you are trying to use the transistor as a switch. To do that, you want to drive it into saturation. That'll never happen with a base resistor of 1.5 meg. You want to drive the base with as much current as you can, with the following absolute limitations:

1) do not exceed the maximum allowable base current, and 2) do not draw more current from the source than it can provide

In practice, you want to drive the base at some "sweet spot" above the minimum that works reliably, and below say 1/2 the smallest absolute maximum in 1) and 2) above

Your math is misapplied. With the pushbutton pressed, the circuit to which it is connected draws .65ma through the resistance of the pushbutton. That resistance is an extremely low value, and the Vd across the pushbutton is also extremely low. Your math will limit the current that can be drawn through the transistor Ice to .65ma (assuming your numbers are correct - I didn't check) when you really want that current to be limited only by the load. Essentially, if your math is correct, you are making the transistor simulate a resistor of about 15K ohms when you really want it to be a short circuit.

Stuff a 4.7 K in the base. That'll limit the base current to around 1 mA, and turn the transistor on harder. That may fix your problem. I don't know how much current you can safely draw from the digital source - depends on the source itself and any loads already on it - but my guess is you would likely be safe reducing the base resistor to 1 K if the 4.7 K doesn't work. Nevertheless, I wouldn't go below 4.7 K without verifying that the source can safely provide more than 1 mA.

Reply to
ehsjr

hey Kirk....

I've embarked on similar projects with similar effects....

In my case it ended up that the digital circuit cotributed enough RF noise to the system that this reduced the range dramatically!

Find a way of seperating the digital from the rf electrically or use good EMI filter components....

good luck

Reply to
weco

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