I’m using the MH-ET Live Scanner 3.0 with an ESP32.
My application uses the scanner to read a QRCODE. If the QRCOde is valid, I activate a relay, a simple application…
I power the scanner through the VCC and GND pins and use the TX and RX pins for serial communication. Everything works correctly, but I have serious problems with the 5V power supplies. The context is as follows, I use a 5V power supply to power the ESP32 and the scanner. When I power the processor with a market standard power supply, the scanner stops working correctly, it keeps beeping and the red LED lights up and this prevents the reader from working correctly, the scanner is not able to read the qrcode. If I power the esp32 via the laptop’s USB communication cable, the system works correctly. If I power the esp32 with a 4.5V battery the scanner also works correctly. Using a standard Samsung source (usb output with P4 standard output) the scanner also works correctly. I did a test using a 5V Isolator DC/DC Converter - model B0505S-1W and with this component the scanner works correctly, however the B0505S component only allows currents up to 200mA and I use 450mA (260mA from the esp32 and another 200mA from the scanner).
I would like to know if anyone has experienced this and what solution was used to solve the problem.
Thank you ,
While I have no experience with this particular device, your symtoms (works on USB connection but not with an external 5V supply) suggest that you have a power drop problem. It sounds to me like that the 5V to the scanner is dropping below 5V and causing the scanner to malfunction (very possibly because of a large current spike when some internal function starts.) This is a rough drawing of what I think you have
I am assuming that the power wires to the scanner (circled in green here) are fairly long. I believe what is likely happening is that a current spike when something in the scanner starts up is causing the 5V at the scanner to drop which causes the scanner to malfunction. There are several ways to solve this (getting progressively more expensive!) first a bypass capacitor at the scanner power supply like this:
here if a short term current spike occurs the initial current will be provided by the tantalum capacitor until it can charge from the 5V source (this assumes that your 5V source can provide sufficient current to power both the ESP and the scanner, another thing to verify, increasing the current capacity of the power source may also help!) If that doesn’t do the job a more expensive solution would be two buck power supply regulators like this
Here the power supply has increased from 5V to 7, 12 or up to 36V in to the two buck regulators. Their output voltage is adjusted to 5V (you need to do that before connection the scanner end ESP32 to them to avoid damaging them!) and provide independent 5V regulated sources (preferably with short wires) to the two devices. I expect the bypass capacitor in the first drawing will probably work though assuming the problem is as I suspect a drop in the supply voltage to the scanner (which seems likely because the USB connection which does much the same as the buck regulator) works. Hope this helps.
Thanks for your help friend, but I accidentally discovered what is happening, it seems strange but the problem occurs when I use power supplies that deliver more voltage than 5.2V.
To discover the problem, I used an original power supply from a good quality Samsung cell phone, and the problem did not occur. I analyzed that the output voltage on the Samsung power supply was 5.1V and when I fed the circuit (esp32 plus scanner) the voltage dropped to 4.78V. So I measured the voltage on the sources I used in the tests and they were all delivering more than 5.2V (one of them delivers 5.6V).
So I used an LM2596S Step-Down adjustable voltage regulator module and set the output to 4.5V. I tested this change with several power supply models and in all cases the problem was resolved.
I really appreciate your help.
That does seem odd! Normally a 5V device will accept up to 5.5V of so without problem. The data sheet I found on the net doesn’t have any information on what voltage range the device will operate in, but at least you found a solution.