# Hantronix 20x2 LCD backlight

I recently got a big box of Hantronix 20x2 LCD from a friend. They appear to be the negative type, where the 'on' pixels appear as clear and the off are opaque. When he gave them to me he told me that there was no backlight, however it appears that there is something connected to the backlight terminals, and I'd like some help identifying it.

My best guess is some manner of EL panel? If it is a backlight, can someone tell me how to go about enabling it?

I've tried 3v3 across the terminals, to no effect. In case the pictures didn't turn out, the alleged backlight is two flat leads leading into a laminated plastic device sandwiched between the LCD body and the circuit board backing. I've also included a picture of the back connections, in case someone can spot a missing resistor, perhaps?

• That light pink plastic sandwich is an EL panel. @OliGlaser's answer beat me to my "Post your answer", he's given you the right way to illuminate it. There are several inexpensive "EL wire + battery driver" options on eBay, just use the driver and send me the EL wire. :-) – Anindo Ghosh Feb 8 '13 at 4:41
• @AnindoGhosh - that's a nice alternative if a ready built driver is required. Can't grumble at $4.50 (+$3 shipping) for both wire and driver. – Oli Glaser Feb 8 '13 at 4:49
• @OliGlaser And I get some free EL wire out of the deal :-) – Anindo Ghosh Feb 8 '13 at 4:57
• Excellent, thank you everyone! I'm going to try my hand at building a driver, doesn't seem overly complicated, but I'll look at the eBay route as well. Thanks! – Chris Feb 9 '13 at 5:45

If it is an EL backlight, then you'll need a "high" voltage AC source, typically around 100V-400V at 50-400Hz. It's a tiny current, so the light is pretty efficient despite the high voltage.

Only trouble is it's a bit more complex to generate the voltage, this link shows how to build a basic boost converter for an EL backlight. If you google for "EL driver" or similar you will find more examples:

In the above circuit, Q1 switches on and current flows through the inductor. Then Q1 switches off, causing a high voltage to appear at the drain of Q1 (charging C1 through the diode). Q2 then switches on to put the voltage across the 100kΩ resistor. 2 Timers on an AVR are used in this example. You can adjust brightness by varying the frequency or duty cycle on Q1. Q2 controls your AC frequency.

(just in case - if it is a standard LED, you would need to apply the +V to the A, and ground to K. If it's a white LED with >3V drop then at least 4V may be needed with a series resistor to limit current. However, looking at the back it seems the jumpers for what may be the LED option are not connected, and as Anindo says it does look like an EL panel poking out under the display)

According to (what I think is) the datasheet, it seems there are various option for the backlight.

• are some special high voltage mosfets required or can simple be 702s used? Also, what are instantaneous current requirements for a power source? – miceuz Feb 8 '13 at 7:54
• @miceuz An EL panel electrically behaves like a capacitor: Feed it sine waves and instantaneous current requirement is not high. Feed it square waves, and it will draw current spikes. The specific current draw can only be determined from the datasheet (which apparently does provide this info in this case). – Anindo Ghosh Feb 8 '13 at 14:08
• @miceuz - Anindo covered the other part, the current is tiny (sample datasheet) If you mean 2N7002s, they wouldn't be suitable due to the 60V max drain-source rating. The MOSFET can be small, just needs a high Vds rating. – Oli Glaser Feb 8 '13 at 14:22