Help With Schematic

[AustinTxBob]

For my digital fundamentals class, I had to build a kit.  Instead of building dice or something like that I decided to go with something I can use so I went with a small frequency counter to be used with a HF transceiver.  Problem is there was no block diagram and I need to do a write up on how the thing works.

From what I can tell, all of the counting and driving of the LEDs is done in a programmed chip (PIC probably but they have a label covering it up).  The input into the chip is what has me confused.  A 2n3904 and 2n3906 are used in what I am guessing is an amplifier configuration (class a?).  Then that is fed into a 74hc02 which then goes into the programmed chip.  I think the NOR gates are there so the programmed chip only counts pulses during certain times (probably driven based on the 4MHz crystal).

I haven't put this on the scope yet to see what is going on and was hoping someone could tell me if I am on the right track or not.  I can't find any transistors configured exactly in this way.

Thanks.
Bob

[Zero999]

Yes the BJTs are configured as an amplifier. Think about what happens to the collector voltage on Q1 when the base voltage changes, note that it's connected to the base of Q2 and do the same there. Then think about what happens to the collector voltage, when the voltage at the emitter of Q2 changes. Hopefully you'll see that the emitter resistors and R25 provide negative feedback, which reduces the gain but brings about many advantages: look them up.

The 74HC02 is configured as  a flip-flop. notice how it's drawn as AND gates with circles on the inputs? The circles denote that there's an inverter before each input. Look up DeMorgan's theorem and you'll find they're NOR gates.

Did the kit come with source code for the MCU? If not then it's not very good. The whole point of building a kit is to learn how it works!

[Towz]

Looks an awful lot like some kind of dedicated frequency counter chip like the intersel 7216b. Not a uC.

[AustinTxBob]

Yes the BJTs are configured as an amplifier. Think about what happens to the collector voltage on Q1 when the base voltage changes, note that it's connected to the base of Q2 and do the same there. Then think about what happens to the collector voltage, when the voltage at the emitter of Q2 changes. Hopefully you'll see that the emitter resistors and R25 provide negative feedback, which reduces the gain but brings about many advantages: look them up.

The 74HC02 is configured as  a flip-flop. notice how it's drawn as AND gates with circles on the inputs? The circles denote that there's an inverter before each input. Look up DeMorgan's theorem and you'll find they're NOR gates.

Did the kit come with source code for the MCU? If not then it's not very good. The whole point of building a kit is to learn how it works!

No source code.  They're pretty tight on handing out info.  Can't even find a manual online. :/ 

Not sure why I didn't realize it but of course the transistors have to amplify the signal.  The NOR gate needs logic voltage to work.  The output from the transistors seems to be a 0-3v square-ish wave.  That makes sense.

I'll do some research on exactly how the transistors are doing that.  I need to try and wrap my head around what exactly they're seeing and doing.

Looks an awful lot like some kind of dedicated frequency counter chip like the intersel 7216b. Not a uC.

I'm pretty sure it is a custom program they're running.  It has several programmable features like blanking the display, memories for storing offsets in the case of connecting to an IF stage plus some other options.  It also goes to 50MHz.