Hello,
I am about to send away my board to get some manafacturerd and partly assembled.
The board simply takes the pulse from an RC receiver, and uses an AVR to turn on the transistor, which will turn on a whole load of LED’s.
The voltage regulator is 3.3v for the AVR, LED’s run off the battery, which is 6v.
Please tell me wh you think, and if there is any silly mistakes.
http://habnetwork.com/Origial%20Board%2 … ematic.zip
Kind Regards,
Angus
oh and just incase some think im a spammer, im not…
i have posted a before here… once…
Cheers,
Gussy 
What program did you use to make it?
Kuroi Kenjin:
What program did you use to make it?
Was done in eagle (looks like)
It was done in eagle.
Can someone please look at the schematic, just to make sure i havent done anything stupid?
only things i can see that might cause issues is the decoupling caps and the fact that they are on a seperate part of the schematic. When they’re done like this, its a pain in the rear to get them to connect to the pins themselves (at yes, schematicaly its the same, but they’re meant to be close to the pins of the IC, not just anywhere on teh board), at least this is what i’ve experienced, it doesnt draw the ratsnest the way one might expect.
also, why the .1µF on the input supply? If its meant to do what I think its meant to do, it should be a little larger, no?
the .1µF and .33µF are straight off the data sheet, i belive that the .33µF is to stop the regulator oscillating… i will check the values with the datasheet again.
Also, say i want to drive 24 LED’s, which are all 3.3v led’s, and my VCC is 3.3v, i do not need any resistors then do I?
Thanks,
Gussy
If those values are from the datasheet, then by all means, leave them.
As far as the resistors, they are current limiting resistors, so the LED’s dont draw too much current and burn up.
The transistor you’re using has a max current source/sink, I’m not sure if just relying on the transistor to not source/sink more than that is enough, or if the LED’s will just keep sucking, and blow the transistor and or themselves. I would guess your going to need some resistors somewhere along the line.
gussy:
Also, say i want to drive 24 LED’s, which are all 3.3v led’s, and my VCC is 3.3v, i do not need any resistors then do I?
Gussy,
If the LEDs are connected in parallel you will need ballast resistors to stop the lowest voltage LED from hogging the current. On the other hand, if the LEDs are in series you’ll need a lot more voltage (24 x 3.3 = 79.2V). A more practical solution is to arrange the LEDs in series strings of a few LEDs that in turn are in parallel. Each series string would need its own limiter resistor. This is common in cell phones and other keyboard lighting or LCD backlighting with LEDs. There are many step-up converter ICs to run the LEDs strings from, say, 20V derived from a single 3.6V Li-ion cell.
Each LED has a forward voltage drop that depends on brand, colour, and manufacturing tolerances. Where are you reading “3.3v led’s”?
Why are you dropping the VBATT to 3.3V for the AVR? A tiny25 can operate directly from 5.5V (6.0V max), and at higher supply voltage has lower port output impedance (ie greater drive ability).
The BC817 driver transistor (on your schematic) will have a finite saturation voltage, which reduces the voltage available to the LED(s). For a BC817 this is 700mV max, but good design must account for a 700mV drop at the LED output pin. Less in practice due to contact and wire resistance voltage drops.
Next, the BC817 has a max dissipation of 250mW, so at the 700mV VCE (sat) spec the max collector current is 257mA. The device is limited by its thermal rating in your design.
The BC817 is drawn as a common emitter amplifier on your schematic with the base current limited by the 180 ohm resistor, and the output impedance of the AVR port. A tiny25 running at 3.3V can only source 5mA, with about 800mV saturation at either high or low logic level, meaning you only have 5mA max at 2.5V output to drive the transistor. The 180 ohm resistor is meaningless, the BC817 will saturate with 5mA of base current.
It’s not good to run the AVR ports into a heavy load. Increasing the resistor to 360 ohms will protect the AVR in your design. At 5mA base drive the BC817 will produce 500mA LED current (assuming an hfe of 100 min), but will still overheat as noted above.
Some how you have to account for the difference between the LED Vf plus the transistor VCE(sat) and the VBATT voltage, which is the role of the LED series resistor(s).
Conclusion.
(1) I wouldn’t build this PCB before building a prototype with the actual LEDs you plan to use. The design might work as drawn, but several improvements could be made first.
(2) A PMOS FET is a much easier device for power circuits (such as 24 LEDs regardless of connection). FETs are rugged and available in high current ratings (tens of amps or more). Finding one that will operate directly from an AVR port on 3.3V supply is tougher. A FET driver stage, between the AVR and the FET, with operation from the higher battery voltage is probably the better solution to your project.
Comments Welcome!
Peter