First PCB order. Requesting Advice on Electrical Schematic.

The 78L33 is not a suitable regulator for this application - you are taking power from a USB connector using only the power pins (itself rather bad practice) so you supply voltage may be as low as 4.5V according to specs but down to 4V in real life. The &*L33 (page 4 of ST's data sheet) has a dropout voltage (max) of 1.7V so it needs a supply of at least 5.03V to remain in regulation. Change the 78L33 for a modern low drop regulator.

You have no small ceramic caps across the supply to the processor - add a 0.1uF ceramic cap to ground for each VCC pin on the processor. They must be placed close to the processor pins. You need similar caps for ISP1.

The ESP8266 will need more decoupling - are you using  a ready made module or a bare chip. Don't use the bare chip unless you have access to, and know how to use RF test gear.

I would expect  a WiFi chip to draw quite large peak currents - on the little module using this chip they use  a much beefier regulator than you have. Check out circuits on the web for these modules and ask yourself why yours is so different.

MK

The 78L33 is not a suitable regulator for this application - you are taking power from a USB connector using only the power pins (itself rather bad practice) so you supply voltage may be as low as 4.5V according to specs but down to 4V in real life. The &*L33 (page 4 of ST's data sheet) has a dropout voltage (max) of 1.7V so it needs a supply of at least 5.03V to remain in regulation. Change the 78L33 for a modern low drop regulator.

You have no small ceramic caps across the supply to the processor - add a 0.1uF ceramic cap to ground for each VCC pin on the processor. They must be placed close to the processor pins. You need similar caps for ISP1.

The ESP8266 will need more decoupling - are you using  a ready made module or a bare chip. Don't use the bare chip unless you have access to, and know how to use RF test gear.

I would expect  a WiFi chip to draw quite large peak currents - on the little module using this chip they use  a much beefier regulator than you have. Check out circuits on the web for these modules and ask yourself why yours is so different.

MK

MK,

Thank you for the insight. I will work on sourcing a better voltage regulator (any suggestions?) as well as adding filters to my ATmega328P-MU, ESP8266, and the 6 AVR pins. Could you shed some insight on what you would do to not take power directly from the power pins of the USB connector as you indicate is bad practice? 

For clarification, the battery terminal is attached to a 3.7V 500mAh lithium ion polymer battery. I am currently using the ready made ESP8266 version 3, but I also have version 7 and 12 that I am tinkering with. I put the voltage regulator in the circuit because the ESP8266 is only rated for 3.3V, but I see what you mean as the ESP8266 will probably draw 300mA at peak, and the 78L33 is only rated for 100mA.

Because the battery nominal voltage is 3.7V, do you think I can get away with removing the voltage regulator altogether? (i.e. How much less time do you think the chip will last? I have seen the ESP8266 work at 5V.)   

Thanks Again

Frank Vigilante

Any suggestions on decoupling the ESP8266? 

You haven't said if you are using the chip or a module. Can you give me a link to the data sheet an I'll see if I can help.

One thing I can tell you - NEVER, EVER, run a chip at higher than its rated working voltage. the National Semiconductors Audio Handbook  (1970s sometime) put it well. Outside absolute maximum ratings all bets are cancelled.

Chips won't all fail instantly (some will) but operation and reliability will be compromised.

MK

Michael,

I am using a module. It is the ESP8266 version 12, but I am also playing with version 7 and version 3. Version 12 supposedly has an ADC on it, which is needed in my application (although it only goes up to 1 volt). I am hoping that eventually the ESP8266 will have multiple ADCs that can handle up to 3.3V...this way I can eventually discard the ATmega chip.

Thanks

Michael,

Also, I have spent some time looking for a suitable replacement voltage regulator. Do you have a recommendation for something that can handle up to ~200mA, and outputs 3.3V with simply wiring? Something relatively inexpensive?

Regards