BJT project/design ideas

[douglaswilliams]

I need to build a simple circuit with a BJT. Any functional circuit will do but I have had trouble thinking of one. I looked at circuits for BJT logarithmic converters, but they are way too complicated and big for me.

Does anyone have any ideas? Thanks.

 

[Torben]

I need to build a simple circuit with a BJT. Any functional circuit will do but I have had trouble thinking of one. I looked at circuits for BJT logarithmic converters, but they are way too complicated and big for me.

Does anyone have any ideas? Thanks.

Have you already covered switching with BJTs? A light (or dark) detector can be made with one or two BJTs, an LDR, an LED, and a couple of resistors. And some wire. Google "light alarm" or "dark alarm" if that interests you (or search this site for them--they've been covered here a few times).

The simplest BJT circuit I can think of off the top of my head would probably be a logical inverter. One transistor, one BJT.

There are more complex "simple" projects too--so the question is "how simple are we talking here?"


Anyway, good luck,

Torben

 

[douglaswilliams]

Yes, something like the light detector, but maybe a little more complicated.

 

[Torben]

Yes, something like the light detector, but maybe a little more complicated.

Can you be a little more specific? What does "a little more complicated" mean to you?--like how many transistors would you like to use? Have you had any other ideas?

What about a simple light intensity meter using an LED bar graph display?


Torben

 

[KMoffett]

Douglas,

How adding a delay and/or hysteresis to the detector so it only responds to long term changes not just a passing bird/cloud/UFO.

Ken

 

[douglaswilliams]

Yes, a light intensity meter using an LED bar graph display sounds great. How do I do that?

 

[douglaswilliams]

or maybe using multiple LEDs in a row as a bar graph, that would be great.

 

[Torben]

or maybe using multiple LEDs in a row as a bar graph, that would be great.

Hi Douglas,

Yep, that's the general idea. That's really all a prepackaged LED bar graph is anyway. Some might have some other support stuff built in but you can get, say, just a bar of 10 LEDs which looks nice and professional. Some will have the first few LEDs green, the next couple of LEDs yellow, and the top one or two red, like you might see on an audio meter.

First you might make up a simple light detector with one BJT to make sure you've got that bit down. This will give an output roughly proportional to the light detected. I made one a week or two ago while messing around in which I used an LED to show the detected light level: it would fade on as input light increased and fade out again as input light decreased.

Then you can take that output voltage, amplify it if required, and feed it to an LED bar graph. You could make your own LED bar graph from discrete LEDs, transistors, and resistors (see the right-hand portion of **broken link removed** for an example).

Take a stab at designing the individual bits and post what you get. People on this site love to help people out, but generally won't design a whole project for you unless you pay them. It will also help if you say what kinds of circuits you've already had experience designing.


Cheers,

Torben

[Edit: Adding hysteresis or a delay like Ken mentioned will help keep the meter from jumping all over the place and give a smoother display if you find that it ends up looking too jumpy.]