Practical PIC Projects

   

Universal Driver & Dev Board

  • Description
     
  • Schematic
     
  • Circuit Description
     
  • PCB Artwork
     

 


 

Description

The simple 8 channel LED chaser has been a very popular project on Picprojects since it was first published a couple of years ago.  During that time there has been a demand for a board that could drive more LEDs so I started designing a new version of the LED Chaser using MOSFETs.  What I wanted to do was come up with a circuit design and PCB that while fulfilling the need for a MOSFET driver could be shared with a number of different projects.  It's always difficult trying to do this without either finishing up with a large PCB or one that isn't much use for anything else.  The final design I came up with is now Picprojects PCB480C and is discussed on this page.


Schematic

Download schematic in PDF

Circuit Description

As the schematic shows the board contains a number of sub-circuits which can be included or omitted as required.

The power supply section uses a 7805 positive 5 volt regulator, the provision of C3 enables the use of an LDO regulator such as the LM2940CT-5 which requires a large (47F) for stability.  Diode D1 provides reverse polarity protection and D14 protects against reverse biasing of the regulator output should the board be connected to an external 5 volt supply.

The board is designed for the 18 pin Microchip PIC16F devices including 16F84A, 16F627A, 16F628A, 16F648A, 16F88* and the new Enhanced midrange parts 16F1826 / 16F1827.  The provision for an external crystal and load capacitors allows designs to use higher clock speeds or more accurate timing over the internal RC oscillator module.  A 5 pin ICSP connection is also made available for in-circuit programming and debugging.

* The 16F88 uses different I/O pins for the ASUART so it can't use the serial hardware interface on the PCB

An Infra Red Receiver module such as the TSOP4838 can be used with the board, the output connecting to RB0 I/O pin.

Switch S1 provides user input and connects to RA4 with provision for a 10K pull-up resistor.

The lower four I/O pins of Port A and the high four I/O pins of PORTB are connected via resistors to on-board LEDs and also to a 16 pin header.  The header allows 8 I/O lines to be used externally via an IDC cable connection.  These same eight I/O outputs can also connect to STP36NF06 or STP20NF06L logic-level N-Channel MOSFETs for driving high power loads.  The MOSFET drains are connected to 5.08mm 16 amp terminal block connections.  Each channel can handle 3 amps, however the combined channel current for the whole board should not exceed 16 amps.

A discrete RS232 level shifter is included allowing the board to connect to external equipment using serial comms.  Since this circuit doesn't have a -12 volt supply it makes use of the RxD input from the host equipment to 'steal' the negative supply for the boards TxD output.  Generally this works well but does rely on the host comms equipment using RS232 signal levels.  The input for the SIO is a 3-pin 2.54mm header.  The centre pin is ground and the outer pins are Rxd/Txd.  This arrangement allows  rotation of the connecting socket through 1800 to convert input connection from DTE to DCE.

n.b. The 16F88 uses different I/O pins for the RX/TX AUSART peripheral to those used by the 16F627A/8A and 16F1826/7 devices so the serial hardware cannot be used with this part.

The final option is the provision of two external input/output connections.  The input mode can either use the opto-isolator or by omitting the opto-isolator and shorting the solder jumpers on the PCB provide direct access to I/O pins for use with digital or analogue inputs.

The opto-isolator does not provide high voltage isolation since the signals are routed in close proximity to other signals on the main PCB.  The main reason behind its inclusion is to provide ground isolation and input protection.

When the Opto-isolator IC is not used the I/O-1/2 connectors provide direct connection to the PICs I/O pins for use as either digital I/O or analogue inputs.  The resistor/diode/capacitor on each connection allows input filtering or output slew rate control.  Pull-up resistors are also provided on these inputs.  These components can be included/omitted, values altered etc. to suit whatever application the I/O pin is used for.

The SJ1 and SJ2 solder jumpers will also take an 0805 SMD resistor allowing a resistor to be inserted between the PIC I/O pin and capacitors C9/C10.

Solder jumpers SJ5 and SJ6 allow the two inputs to be routed to different I/O pins

SJ5 connects I/O-1 connector to either RB1 (shared with the Serial I/O RXD input) or RB0 (shared with the IR Receiver module)

SJ6 connects I/O-2 connector to either RA4 (shared with switch S1 input) or RB3.

At the right hand edge of the PCB next to the two I/O terminal blocks are solder points for access to 5V and Gnd for external use.

Finally, a resistor and LED across the 5 volt supply can be fitted for power-on indication.


PCB Artwork

The PCB is available to buy from the Picprojects online store.  This a quality double sided, thru-plated board with solder masks and component overlay on FR4 board with RoHS compliant OSP finish to the copper.

The artwork is provided if you want to etch your own board, however it is a double sided board and unless you can thru-plate the holes it will be difficult to solder both sides of some components such as the terminal blocks.

 

Artwork bottom Artwork top Component overlay
     

Physical dimensions and centres