Voltmeter display.

[Dave Plowman (News)]

Bought a rather nice small LED voltmeter off Ebay for not a lot. It's a 3 digit type. For the old Rover which didn't have any battery indication - intend fitting it in the middle of the rev-counter face. However, it's a bit bright for night time. Any clues about dimming it? Can't get a data sheet for it - came from the far east. The PCB has a single processor and two regulators - one 5 volt and one

3.3v. I'd guess the latter is for the display. But being surface mount, rather difficult to suss out the schematic.

[Bernard Peek]

Sweet wrapper stuck over the display.

[Frank Erskine]

A bit of neutral density filter material - or even colour gel to suit the car interior!

[Andy Dingley]

Suss out the schematic. You probably only need to find a chip number, then search for a datasheet. This should give you (if it's easy) the identity of the brightness control pin, then you probably need to up a resistor value supplying it.

In extremis, look for mid-range resistors supply an IC pin from Vcc, with no other connections, then experiment.

[Andy Burns]

Well, I was going to suggest some neutral density filter material ...

[Roger Mills]

But I imagine that the brightness needs to be switchable - maybe going to the lower level when the car lights are switched on.

[Dave Plowman (News)]

Should have said its fine in daylight. Only too bright at night.

[Dave Plowman (News)]

In one.

[Dave Plowman (News)]

Tried that - no joy. The PCB is marked JuYing.Ele.cn. ;-) The IC PHL 044. Can't find anything on that unlike the voltage regs.

[NT]

I'd run it from a current limited supply, connect a probe to ground via a 10k resistor, and prod around the circuit. Somewhere it will alter the brightness, failry likely.

NT

[Ron Lowe]

LEDs normally have series resistors to limit the current, ( otherwise, they represent almost a short-circuit in the forward direction! ) and this controlls the brightness.

There may be a resistor bank somewhere near the displays.

THe current in the LEDs will be (supply voltage - 0.7 )/R So dropping the voltage will also dim the displays.

However, that assumes that the displays have their own supply which does not also supply ICs. If you drop the voltage to the ICs, then it will stop working.

3.3 may well be the the IC supply, and the 5v for the display. You'd need to trace it out on the PCB. If one supply does indeed supply the displays only, then yes, you can rig up a dual-voltage arrangement, and have the vehicle lighting toggle which voltage is used.

[The Other Mike]

Do you really need an absolute value - not that long ago the LM3914 coupled with a few LED's was enough to satisfy the needs of endurance racing at Le Mans. Dimming is easy to achieve, voltages above and below normal can be made to flash the display.

Rather than use a 10 LED's you could even substitute a single multi colour LED.

[Dave Plowman (News)]

I do have a circuit for a similar device - a Vellerman LED voltmeter. There are no external series resistors on that - just a driver chip. The common anode on each display is fed via a transistor controlled again by the driver.

The PCB is surface mount and fitted to the display. It is at least two layers, so not possible to trace it out without removal.

There are very few external components. The 5 volt reg is much smaller than the 3.3v one, so my guess is 5 volts for the IC, 3.3 for the display. Since it's a blue display, 3.3 could be used to drive it with no current limiting resistors?

My thought are to replace the 3.3 reg with a variable one and try a lower voltage? There's plenty space to add an extra PCB in this installation.

Unless anyone has a better idea?

[John Williamson]

For that matter, I recently bought a dual output power socket for about a tenner with an LCD DVM built into it. The power supply I use for the netbook has a backlit one built in, and that was only about twenty quid.

Not much help to the OP, though, as he wants to use what he has.

[Dave Plowman (News)]

I'm not racing at Le Mans. ;-) An accurate voltage readout tells you about the state of several things. Battery condition with the engine stopped, charging system when running.

Ages ago I fitted a current measuring system with a an LED which is green on charge, red on discharge and amber when balancing the load. Which is ok as an idiot light, but not so nice as a voltmeter.

[Dave Plowman (News)]

There are some gotchas with these meters. Not all can be used to measure the voltage of their own power supply - like a car battery. There are small LCD units which can - but I couldn't find one with illumination.

It's to be fitted in the analogue rev counter dial. The numbers on that are approx 5mm in height. The LED display I'm using, 10mm. Most are a great deal bigger.

[Jules Richardson]

How are the LED segments driven? 3-digit is 21 segments (ignoring DP) - are each of those controlled by a single pin on the IC?

[Andy Dingley]

Course they can - you drop the supply voltage with a big linear reg to

5v or 3.3v as needed and you divide the measured voltage down to about half of this. They'll thus work quite happily, as you're not trying to measure a voltage anywhere near the supply rail voltage, and you've a stable voltage to drive the meter circuit. If the battery voltage drops too low to drive the meter chip (on the far side of a linear that's expecting to be halving it), then your battery is flat and you've probably noticed this by now.

Otherwise just rewire your car with a commercial DC-DC module inside the dash and have cleaned-up 12V & 5V supply rails available to feed anything you hang in there. My MG's dashboard even has a crankcase accelerometer to tell me to back off from breaking the crankshaft - output is a brown-backlit LCD text display which just looks like a bit of art deco bakelite when the power is off.

[Dave Plowman (News)]

It's more to do with the input to the module, IIRC. Some certainly say they aren't suitable for measuring their own supply.

But there's no need for any of this with the cheap unit I've bought. It works straight out of the box. All I need to do is find a way of dimming the LEDs when the car lights are on, at night. ;-)

[Bob Minchin]

Most LED displays are multiplex driven 7 or 8 rows x 3 columns and are bit of a pig to dim unless the driver has a pulse width modulator built-in which many do. Single resistor pull up or down usually sets brightness. With no data on the chip then careful probing with a 10k resistor is possibly the way to investigate.

Bob