Ebay Voltmeter with negative voltage reading capability?

[rfengg]

Ebay is rife with 3 terminal voltmeters that can read voltages which are higher than the  negative terminal of the voltmeter and they work well for the price.

Here is my problem : I have a +3.3V supply and a -2.5V supply with a common ground which are derived from +9V and -9V respectively using voltage regulator IC's and would like to monitor these supplies using these Ebay displays.
I have connected the +3.3V supply to the sense terminal of the aforesaid 3 terminal voltmeter with the Vcc of the display being connected to 9V from where the 3.3V is derived and all's well.
To monitor the -2.5V rail, when I connect the -2.5V to a second voltmeter sense terminal , it shows zero as it cannot seem to read voltages that are are lower than its negative terminal.
I tried connecting the -9V and ground terminals to the negative and positive terminals of the display but then when I connect the sense terminal to the -2.5V , it reads 6.5V as it should.

Are there cheap LED meters (below $5) that can indeed read negative voltages as well? What other alternates do I have that can fix this issue?

[ledtester]

Have a look at this ICL7107 ammeter kit:

https://www.aliexpress.com/item/4000452559613.html

Here's a schematic:



It has everything you need for a meter including voltage reference, display LEDs, mounting bezel and -- importantly -- a charge pump to generate a negative rail so you can read negative voltages.

It is set up as an ammeter, i.e. to measure current. but it should be easy to modify it for voltage -- perhaps by just removing R5.

You can adjust the input range by choosing an appropriate voltage divider for the input. Have a look at this ICL7107 voltmeter project:

http://www.electronics-diy.com/ICL7107_volt_meter.php

It's only 3 1/2 digits, but it's also pretty cheap.

[NiHaoMike]

Another way is to use an inverting opamp to turn the negative voltage into a positive voltage.

[magic]

There are panel meter modules with ICL7106 (LCD, sometimes backlit) and ICL7107 (LED). They are a bit more expensive and not as common, they use a total ~9V supply and have differential inputs, but not rail to rail (~1V on each side IIRC). So you would need to power it with ±5V, perhaps. They measure up to ±200mV natively or 10x/100x more with a divider, often included on-board, sometimes easy to adjust. It's also important that the decimal dot be adjustable too, or it will look weird.