Hello again. I'm the bloke who was on a few months ago worry about his frequency-voltage converter. Which now works magnificently, and we have one wind tunnel up and running.
So - on to Mk II. I intend to put all the components on one circuit board, with one power source: a regulated 9v DC supply with a max power of 350mA.
The IR LED/sensor works at 4.5v, the f-v chip at 9v. Is there anything to stop me from slapping down a variable resistor and taking the 4.5v from it?
If not, how ought I calculate the value of R I need? If yes, how should I do it?
Simon Morden
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You forgot an important piece of information: The current required by the IR LED/sensor. So I can only guess it to be some tens of mA with some variations depending on the status of the components and the environment.
A resistor is not the way to go as it cannot provide a reliable, stable voltage. A variable resistor is worse as the common types, trimpots, can dissipate only a few mW so you may fry it pretty fast. The quick and easy way is using an 7805 type voltage regulator and one or two capacitors.
if that is a integrated IR sensor, using a R to drop the voltage may make it unstable. Try using a 78L05 regulator with a small diode in series on the output to give you a ~0.5V drop and place a small cap like a 0.1 uf across the diode..
Many thanks to yourself and Petrus - it's not just working out what the question is, it's working out whether there's a question in the first place!
My local supplier doesn't do the National 78Lxx, but does do the TS78Lxx series. The IR sensor draws 50mA, but will work just as fine off 5v as it will 4.5v - different resistor required is all.
Again, thanks for all your help.
Simon Morden
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Visit the *all new* Book of Morden (www.bookofmorden.co.uk)
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This is called a voltage follower configuration. The base is connected to the pot, the collector to the 9V supply, and the emitter is connected to the positive (4.5V) supply input for the sensor. The current you can pull from the emitter is like
100x the current you can pull from the pot without perturbing the voltage significantly. You can adjust the pot to the appropriate voltage.
One problem is that if the 9V supply is not regulated, you can get a proportional droop in the output, so using a little regulator like petrius mentioned could be better. You might also use a 5.1V zener like this:
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