yampp-7 EL-Backlight

You can order the assembled board for this project at the yampp Web Shop.

Quick Features:

The yampp-7 EL backlighter is finally here.
It consists of a super small EL-driver module and a cool blue backlight foil that suits the Nokia 3310 display perfectly.
The current consumption is about 10-20 mA, depending on how bright you want it to be. The brightness is easily controlled by a resistor on the driver board.

yampp-7 with backlight on (click image for large version)

The EL backlight is available in the web-shop as an assembled unit only.
This is due to the size of the HV857 chip and components and the lack of markings on these. It is available together with a blue EL-foil (as shown on the pictures).

  Circuit Description

The EL-backlight driver is based on the HV857 chip from Supertex. The HV857 High Voltage EL Lamp Driver converts the 3.3V DC input voltage to a 190Vpp AC voltage suitable for driving an EL lamp or EL foil.
The backlight schematic is basically a direct copy of the application schematic from the HV857 datasheet.
The resistor values for R1 and R2 determines the operating frequencies of the EL lamp and the converter. With the values shown, the EL lamp frequency is about 400Hz and the output voltage is 190Vpp.
The current consumption with these components (and the supplied foil) is about 20 mA. This can be reduced, if so needed, by modifying (raising) the values of R1 and R2, but brightness will then be lower.


The EL backlight kit available in the web-shop(Click to see high res. version)

  Partslist & Schematic

Part     Value          Package
-------  --------       ---------
R1       1M             R0603
R2       560k           R0603
C1       3n3 (100V)     C0603
C2       100nF          C0603
D1       BAV103         SOD80
L1       220uH          L1812
IC1      HV857          MSOP8


The installation in yampp-7 is pretty easy.

Step 1

First you need to remove the T2 transistor on the yampp, so you can get the full logic output from the I/O port.
To get the I/O signal out on the display connector, add a small wire between the B and C terminals where the transistor were mounted. (See the picture at the right for details).
T2 modification (click image for large version)

Step 2

Then you need to find a spot to place the EL-driver. As you can see on the image to the left, I placed the driver just beside the display connector, and in direct connection to the EL-foil.
Three wires needs to be drawn to the EL-driver from the display cable - VCC (pin 1), GND (pin 6) and BLIGHT (pin 9).

That's all there is to it. Remember to set the correct backlight mode in the menu on the yampp-7.

You can order the assembled board for this project at the yampp Web Shop.