PBM design

Here follows some hints and practical points, how to assemble the PBM. As usual, when it comes to amateurastronomical projects, you have to put your own personal experiences into it. I provide the concept, you use your own skill and abilities to make it come true.

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Fig. 1. The controlbox.

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Fig. 2. The interior of the PBM controlbox.

I refer to the electrical diagram shown in Sky & Telescope, February 2001, page 139. What is obvious is that I don't care much about appearences. I prefer function.

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Fig. 3. R2 is a 10-turn potentiometer.

The LED forward voltage varies significantly. That is why one needs a broad range of resistance to cope with various manufacturer“s LEDs. And, when using the PBM, one needs a high resolution in setting the LED“s current.

 

The controlbox is a plastic one, very easy to drill holes in. It has a suitable compartement for the 9 Vdc battery. From that space comes red (+Vdc) and a black (-Vdc) wire. The red wire goes to the the main switch (S1). The black wire goes to the bottom of the variable resistor (R2). The red wire goes to two resistors. One (R3) feeds the collector of the transistor (Q1) and the other (R1) feeds the variable resistor (R2) on its top.

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Fig. 4.  A close-up of the main electronic components.

The base of Q1 is soldered directly to the wiper of R2. The emitter of Q1 goes, through the twinkle switch, to the LED via a small connector. (The LED and S2 have switched positions in my box. Does“t matter, they work in series). The return from the LED routes to the µA input of the Digital MultiMeter. The return from COM of the DMM goes to the black wire (-Vdc) back into the battery compartement. If you get a minus sign on the DMM, it doesn“t matter! We are only interested in the numbers.

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Fig. 5.  The panel with the various connectors.

These are not critical to the function of the PBM, but if easy to assemble in the dark? Great!

The purpose with S2 is to intermittent (when you push S2) cut the current to the LED. OK, one could do the same function with S1. But S2 works with a light touch, which is better in the dark.

To obtain all the following voltages, you must shorten +DMM to -DMM of the controlbox. This wire has the same purpose as the current meter (DMM) in place. I assume you have only one DMM to play with. With a fresh 9 Volt battery and S1 in "short circuit" position it should work. Beware, you need a dark environment! If you don't get any light output from the LED, one can check the circuit by measuring the voltage from -Vdc (black wires) to the connection point of the two resistors (R1, R3), +Vdc. It should read the same 9 Volt you measure on the poles of the battery.

If one let the wiper of R2 (also the base of Q1) be at the maximum resistance value, you should read from -Vdc to the wiper, about 2,8 Volt. The LED should be clearly visible. You also could measure, from -Vdc to the emitter of Q1, some 2,1 Volt.  My LED draws 2,9 milliamps in this set-up. At all times S2 is switched ON. If you read 2,6 Volt or higher and no light from the LED seen, the LED should probably be connected in reverse. If the emitter voltage of Q1 is anything else, the transistor is not working correctly. Check the orientation of the collector, base and emitter again. The difference between the base and emitter of Q1 should be 0,6 - 0,7 Volt, if working properly.

 

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Fig. 7.  The DMM.

It has a 200 µA current range, with a resolution of 0,1µA.

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Fig 6. The LED (~525 nm) and the green filter, Wratten 58.


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Fig. 8.  The sighting tube.

The design of the interior of the tube must be up to you. It depends on what material you are going to use. My solution is a couple of sliced plastic pipes (along the lengths) that fit in each other, to house the LED and the green filter.

 

Feel free to comment my design.

Good luck!
Göte

Design 2001-01-10