Hello fellow MCLers!
I decided to use the transistor checker project to learn how to use Kicad and JLCPCB. It was a big hurdle, but I learned how to draw a schematic and make the PCB in Kicad, and was proud when I finally got my circuit in the mail.
Now all the pieces are in, and it’s not working right. A PNP transistor shows red,blue,red as it should, but NPN transistors show nothing. I diode-checked the leds and they all light up with red and blue as they should. I looked for poor joints and bridges, but found nothing. I made some changes from MC’s schematic, but I don’t think they should affect the function of the board. Help?
If that link doesn’t work, try:
Oh, and since this is (obviously) my first PCB, any constructive comments are welcome!
Hi, even though the circuit is fairly basic anything could have gone wrong at this point considering you have no experience with creating boards: first, regardless of the project, I would have made the traces twice as long. traces shouldn’t have those small curls/ wiggles that you’ve introduced (for some unknown reason) from place to place
You have to keep in mind that trace length does matter 9 times out of 10 !!! The more lengths on the threes you have, the more resistance (under load conditions) you’ll have.
The rule of thumb : the shorter (withing reason) the better !!!
Also keep in mind spacing between traces.
Another consideration about trace spacing is that the closer to each other the more (so called) “Stray capacitance” you will have and yes, unwanted capacitance in any circuit could create all sorts of unexpected effects such as transistor oscillations, etc, etc.
C1 (the so called decoupling capacitor for the IC) you’ve connected it wrong ! The negative capacitor pin should have been connected individually and directly to the IC’s negative pin separate from other IC pins also making sure you keep the lead length as short (within reason) as possible or else you defeat the whole point of having a decoupling capacitor there in the first place. for clarification please compare the original PCB design with yours.
Why is not working as it should ? Hard to tell just by looking at the PCB design but make sure the polarity on all transistors is correct and that you’ve fitted the correct value resistors in the right place. Also verify unwanted solder bridges especially at transistors or IC pads as I see that the transistor pads are way closer together than the original design
Hi, even though the circuit is fairly basic anything could have gone wrong at this point considering you have no experience with creating boards: first, regardless of the project, I would have made the traces twice as long
Sorry, i meant to write ‘thick’ NOT ‘long” 🙂
@ovi4 Thank you for the thoughtful response. I used what I guessed was the default for Kicad for the traces. When I got the pcb’s I was surprised as well by the skinny traces – removing a component tended to destroy the trace (glad they gave me 5!). The squiggles are just me not being careful enough.
As for C1 – I guess I’m still learning about electricity. I thought that as long as the connection is made, one route is the same as another (e.g., the pins that connect to the negative cap end are the same). But actually, I don’t really see a difference in how I connected the cap – to the pin – can you explain?
Okay, back to troubleshooting. This project is more full of learning than I expected!
UPDATE: I was troubleshooting (taking measurements, just trying to understand), when it suddenly started working! I realized that my fingers had brushed up against the pins of the IC on the back of the board. After a little experimentation, here’s what I can say:
If I plug in a NPN and it doesn’t light, I can briefly bridge IC pins 2 and 3 and it will light. The timing matters. It may be dim, or flicker off, but if I time my swipe right (???) the lights stay on. I can then plug in other NPNs and it will work fine.
If I then plug in a PNP, it will not light. But I can do the same swipey thing (again, pins 2 and 3) and get the PNP lights to come on. I can then plug in other PNPs.
Clearly, I have magical powers 😏. Seriously, I would love to know what may be going on. Could I be somehow priming something over a threshold of some kind?
RESOLVED: My mini-traces must have not been connecting and so my finger was ‘resetting’ the oscillator. I’ve done some point-to-point surgery and all is well! I’m guessing that one reason for the LED flickering is that the oscillator is going back and forth. When it wasn’t working, the LEDS were solid, not flickering, which might have been a clue?
@runthenclimb There are a number of possible problems with your circuit: 1: cold soldering joints (as you probably) didn’t use enough flux while soldering the components or dwelling time was to short. 2. you might have mixed up the transistors legs when soldering them or even (by mistake) have soldered a PNP in place of an NPN and the other way around. 3. verify your resistor values !. 4. make sure C2 and R2 are indeed of the right value as they decide the osculations value/level/frequency. Also make sure that the DUT (device under test) has its pins connected the right way around but especially the base. Keep in mind that the Base Emitter Collector pins are not positioned the same on every transistor. There are manufacturers that like to have the base in the middle leg, others put the base to the right leg or on the left. Very important. ! make sure that the circuit is fed properly with either a battery or a clean power supply.