Simple circuit to detect 400mV that is isolated from Ground/Vcc powering it? - Page 1
Forum section: electronics / beginners

author: Orihalcon
author_rank: Contributor
timestamp: October 23, 2024, 08:27:34 pm
content: Hello all,I am looking for some suggestions for a simple as possible circuit to detect the presence or absence of approximately 400mV across two points, but I also need the "Voltage detecting" component to be electrically isolated (or at least be very high impedance) from from the ground and 5Vcc that powers the detecting circuit.I then need to take the resulting output of the "voltage detection IC" and feed it to a different IC that gives an output only when a certain period of time goes by where the state of the voltage detection IC remains unchanged.Basically what I'm doing is hooking the "Voltage detection IC" up to single pixel on a figure 8 LCD (the old kind like solar calculators used to use) and detecting if that pixel is on or not. By also checking if that LCD segment state changes at least every 5 seconds, it will tell me if the display has stopped moving as this digit counts seconds elapsed.The system is powered by 5VDC, but the LCD portion of display (at least the two points I'm measuring) does not have a ground or Vcc in common with the incoming 5V power.Ideally, both of these two ICs would draw relatively little power as well and operate on the 5VDC available. Since the voltage we are probing is only meant to power a low voltage LCD pixel, I can't have the detecting component draw too much current as it detects either.Any suggestions? Thanks in advance!

author: PGPG
author_rank: Frequent Contributor
timestamp: October 23, 2024, 10:03:22 pm
content: From what you write I can't get if on detecting side you have any supply voltage or not.

author: Orihalcon
author_rank: Contributor
timestamp: October 24, 2024, 12:34:00 am
content: I think what I want to do can be achieved at least on the sense side with an operational amplifier or voltage comparator - but what has me confused is that most of those circuit diagrams have the ground pins shared when powering the op amp and signal inputs, so I'm not particularly clear on what type of op amp configuration I would want.Since no current actually flows into an op amp input, I don't have to worry about power being drawn from the LCD's pins, but I do worry if I wire it wrong thatI'm also reading there's such a thing as a "virtual ground" with op amps, and that might be key here as well.I'm still not sure what IC I could use to detect "lack of state change over a 5 second period" coming off of the op amp to indicate that the seconds digit has stopped moving?

author: thm_w
author_rank: Super Contributor
timestamp: October 24, 2024, 01:04:34 am
content: Quote from: Orihalcon on October 23, 2024, 08:27:34 pmWhy? Are you sure you can't connect the grounds of the two systems together?If you can't, then you'll need something like an optocoupler, but, since 0.4V is too low to trigger it as PGPG says one option is to steal power from the sensed side. This would power a comparator and an optocoupler, or whatever.Maybe describe a bit more or take some photos, there might be an easier solution available.

author: Orihalcon
author_rank: Contributor
timestamp: October 24, 2024, 03:42:48 am
content: Here's some more information - it's a RM-95 LANC remote for older Sony video cameras. Service manual for the remote is here:https://www.dropbox.com/scl/fi/e4da674gmzwaiayu0qe36/rm-95-Service-Manual.pdf?rlkey=ewa6h0uul5obs611xawjcyxsm&dl=0The reason I can't connect the grounds together is that the voltage at the LCD has a fixed voltage difference from both the 5V supply and supply ground (of 2.3V and 3.15V - not sure which was which when I measured it) and the values at the LCD pins do not change in reference to the main supply ground or 5V supply when the pixel is on versus off. Connecting them I am fairly certain would kill the LCD as it's meant to run on voltages more like 400mV and connecting the to that ground would expose it to at least 2-3V. I think the reason for this non-changing voltage has something to do with the power to the LCD and main IC being buffered from the 5V main supply, but I'm not totally certain on that.I'm thinking there might be some way to use a voltage comparator like the LM393 to look at only the 400mv without exposing the LCD pins to the main supply voltage or supply ground, but I have no idea what wiring configuration would allow for that?Any suggestions definitely appreciated!

author: Andy Chee
author_rank: Super Contributor
timestamp: October 24, 2024, 04:24:05 am
content: Quote from: Orihalcon on October 24, 2024, 12:34:00 amNope. For power supply purposes, you need a real ground. And fundamentally you need a floating supply to power whatever opamp circuit you need to detect your 400mV LCD segment driver output. This detection can then be signalled via optocoupler.For the purposes of prototyping the detection circuit, you could temporarily prototype using a battery as the floating supply. Once the detection is reliable, you can then think about a floating supply, a number of techniques are possible.QuoteThis bit can be accomplished with anything from a monostable circuit, to a microcontroller.

author: antenna
author_rank: Frequent Contributor
timestamp: October 24, 2024, 04:55:06 am
content: Why not tap off the clock signal instead of the LCD digit?LCD's are run on AC signals IIRC to prevent damage to the anode that would otherwise occur running DC into the LCD display. Those signals are really weak too, so jfet or cmos sensing for sure.If you can instead tap off the clock signal and the pin on the LCD driver that goes active when told to start counting, you could just AND gate those signals together, buffer it, then use it to discharge a capacitor in an RC network that otherwise climbs to a given threshold and alarms in the event a clock signal stops ticking. This way you could use the same ground as the LCD driver/timing circuitry and optocouple out from there if necessary.

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 24, 2024, 05:28:53 am
content: As mentioned, LCDs are driven with squarewave AC signals. To "light up" a segment, it is drivenout of phasewith the backplane/common. To turn a segment off, it is drivenin phasewith the backplane/common. What you could do is connect a dual input XOR gate to the segment and backplane/common. It will output a high if the LCD segment is on and a low if it's off. (With bit of glitching which will need to be filtered out) You would power this CMOS XOR gate from the same power rails as the LCD driver (IC4) in the device.

author: Orihalcon
author_rank: Contributor
timestamp: October 24, 2024, 11:51:27 am
content: I'll admit that I'm not super familiar with how these low voltage LCDs work - I just know they draw very little current since a solar powered calculator uses extremely low power and this seems to be of that same type. The 400mV noted came from probing that pixel's input with a Fluke-87V in DC millivolt mode and it gives a consistent 400mV signal when doing that. I think next step is to use an oscilloscope instead to verify that it actually isn't a square wave as mentioned which is probably the case.The RM-95 doesn't actually do the "counting up" part when it displays minutes and seconds - it reports the timecode state that is fed to it by the camera that it is connected to using the LANC protocol, so I don't know that tapping into a clock signal within the remote is helpful.There are arduino projects that can understand the LANC protocol, but I'm particularly curious if detecting stopped timecode can be done with original hardware sensing a non-changing LCD state as I'm not a great arduino programmer either unfortunately. The arduino I'm sure could perform both functions - poll for timecode and then detect when the timecode remains unchanged for a certain number of seconds, and then trigger other things.What specific dual input XOR gate to the segment and backplane/common would be recommended to try that method with and roughly what would that circuit look like?The other thing that I'm able to do is put whatever device can detect the pixel is on "inline" with the LCD if there's a way to sense very low amperages through the LCD - thinking something like a very low voltage "noncontact" ammeter/clamp meter/Hall effect sensor to determine LCD pixel is being lit perhaps? That way maybe it wouldn't require a common ground to the detecting part.

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 24, 2024, 04:10:34 pm
content: I see now that the LCD is multiplexed, so my simple XOR gate won't work for that.You could use an op-amp or op-amps to make a differential amplifier and detect when the peak voltage across a segment is larger to see if it's on.Check outpage 11 of this PDFwhich shows what the AC waveforms could possibly look like on a 1/4 MUX LCD like you have:

author: Ian.M
author_rank: Super Contributor
timestamp: October 24, 2024, 04:23:24 pm
content: An instrumentation amplifier (or buffered differential amplifier for a one off where you don't mind trimming for common mode rejection) will turn those those page 11 differential waveforms across individual pixels into single-ended waveforms you can work with. Use a 'ground' at Vcc/2 to keep the entire waveform within the supplies. Then use a window comparator to detect when the selected pixel waveform exceeds the limits of the de-selected pixel waveform, and retrigger a monostable to hold the result for long enough to bridge the multiplexing frame time. The low cost lineup is probably a quad RRIO OPAMP for the front end, and a quad comparator for the window comparator and monostable.