MC78M05 voltage issues [Solved]

The regulator gives a -1.0v when connected on the PCB and 7 volts at vcc and gnd of ATmega328. is that normal ?

Not only is it not normal it is not possible to get minus 1 volt. So that means your circuit is not wired up like your diagram says. Also you have one resistor for all those LEDs, you need one for each.

Why is the schematic drawn upside down, ground should be towards the bottom and 5V towards the top.

The board was etched with toner transfer method so it cannot be the circuit. Maybe the caps, checked that too. its only when the 7805 is soldered back. The voltage at VCC and GND shows 6v. When without the IC it shows 7v. So the reversed polarity is somewhere else. but where ?

thanks corrected the upside down regulator.

besides the negative voltage the circuit seems to be working except the TVS diode D11 gets a little hot.

I initially had a resistor for each LED then cut them to one as no two leds would light up at the same time and then space was a limited.

besides the negative voltage the circuit seems to be working

So that means your measurement of an negitave voltage is false. You must have made an error in placing the probes.

The board was etched with toner transfer method so it cannot be the circuit.

Don't be silly of course it can. It can be a layout error or an etching error or putting the regulator in the wrong way round, or putting a component in the wrong place, or having a component's pinout not mach the one you have in your cad package.

I hope you are doing these first powerup tests with the controller IC removed.

Post a jpg of your PCB pattern.

I agree with Grumpy_Mike, the toner method is not a reason for errors, the layout of track are.
Home made PCBs can and yours may even have a cracked track that needs a magnifying glass to see.

I would suggest you get your DMM out and check each gnd track for continuity to it various pads.
Then do 5V tracks, then other long tracks, WITH THE POWER DISCONNECTED.

We have been here with this circuit before haven't we?

Tom..... :slight_smile:

Can't thank you all for replies and helping me out here.

below is the layout after i had etched my first one. This had the TVS D11 moved down closer to the Bridge.

I will latter post the pic of the etched board and check the continuity of the GND to all pads suggested by @TomGeorge, yes the circuit is becoming quite familiar :slight_smile: as i lack the knowledge in PCB design though i am Electronics and Communication Engineer but now a Computer Engineer by profession.

so figured out the trace with the short :slight_smile: it was R8 (not in this layout) from my previous design. The leg of the resistor that was in someway in contact with the ground and that explains why that LED wouldn't glow.

next the voltage at the out on pin 3 of the regulator shows a positve 7v which again is quite strange ? Where should i be looking to check on that ? i could see that voltage at the bridge is 10v. The reading was taken without the IC plugged in just a power LED that was on.

Have you got the 7812 in the correct way.

What voltage do you measure between the input and gnd pins, AT the 7812, and the output pin and gnd AT the 7812.

Tom... :slight_smile:

Can you post a picture of both sides of your assembled PCB?

Thanks.. Tom.. :slight_smile:

Yup solved too. It was the GND pin from the regulator that was not connected to the GND of the others.

Missing grounds can cause some of the strangest things to happen - just ask anybody who has tried to get trailer lights working correctly with a missing ground. There does not seem to be any logic as to why which lights turn on when testing :slight_smile: