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Defective PCB Board?

ThomsCircuit

Active Member
UPDATE:
Improved schematic clarity.

This is a simple timer project that turns on a 12V fan for a few minutes and produce a beep from a speaker. The circuitry for the beep are on another board. This board simply receives a low signal from the other circuit. There are 8 pins for the fan and speaker but they are all connected together so multiple boards can be connected to this single board.

I have bread boarded this, made a 2 on universal PCB boards and then i had 5 boards manufactured and built 3 of them as well. They all work except 1 of the manufactured boards. I tested the relay, and changed out the transistor. I tested a few of the resistors before i decided to just make another. I was surprised to find the new board did not function either. It failed just like the previous board. Ive also changed the 555 but no change.

The failure is the relay will not engage and turn on the fan

Could you fellas help me test this circuit to find where the fault is?
Fan Timer-1.png


20220117_133520.jpg
 
Last edited:

ChrisP58

Well-Known Member
Most Helpful Member
Does voltage appear on the two relay coil pins when it should?

If you apply voltage directly to the relay coil, does it actuate? (observe correct polarity)

What is the DC resistance of the installed relay coil? (compare the measurement between the bad board to a good board)
 

ThomsCircuit

Active Member
Does voltage appear on the two relay coil pins when it should?
No. when activated the voltage at the coil is 1.5-1.8 volts. On a good board it is 5.5v
If you apply voltage directly to the relay coil, does it actuate? (observe correct polarity)
yes. like reviving a human from a heart attack it does engage, i then remove the manually applied 5V, it then functions (remains on) on its own via the 555 timer for the designated time indicated by the 1M pot. apx 60 seconds then turns off.

Observations:
there is 5v going to the positive pin of the relay coil
What is the DC resistance of the installed relay coil?
i do not know how to do this. But the positive pin is receiving 5.5 volts
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You could just have some low gain 2N2904 transistors.
The relay needs 80mA at 5V; the worst-case gain for a 2N3904 can be somewhere near 30, and you have just under 1mA base current.

Change R2 to 1K so the transistor has a higher drive current - that should work OK, as long as the transistors are not totally dead or rejects.
 

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