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I have a 433MHz receiver modules (Model number MX-05V) The input voltage is listed as 5V but they don't list the output voltage.

I have seen this used without level shifting in a project in conjunction with an ESP32 module that requires 3.3v signals.

In researching how this was safe I found an answer that suggested that the output voltage would be input/2 but I'm unsure how this or why this is. Unfortunately the answer didn't provide detail, it just happened to mention it in passing. (https://electronics.stackexchange.com/a/103357/274778)

In short, my question is in two parts: What is the signal HIGH voltage (in theory ~2.5v?) for this receiver? Why is it that?

Below is the schematic for the part and a link to a product listing for it, note that on the listing the schematics are labelled backwards.

It's listed as a superhetrodyne receiver and what I take to be the output circuit makes use of a duel opamp (LM 358) but my understanding is too flakey to derive the output voltage from this arrangement.

The schematic for the part

Links to example of part: https://hobbycomponents.com/wired-wireless/1054-433mhz-transmitter-receiver-modules-with-antenna https://hobbycomponents.com/wired-wireless/615-433mhz-wireless-receiver-module-mx-05

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    \$\begingroup\$ Can't tell you from the schematic, not correct/detailed enough. Can't tell you from the datasheet, not linked. SO, impossible to answer. The one thing I can say with relatively certainty is that whoever called this a "superheterodyne receiver" on the hobbycomponents.com website has a lot of explaining to do, and that this is by far the worst schematic I've seen this year. \$\endgroup\$
    – Marcus Müller
    Commented Jan 23, 2021 at 20:10
  • \$\begingroup\$ Like, literally, is this actually the schematic to the circuit you've bought? is there really a 15 pF capacitor in series with a 2700 pF, or did they just show parasitic capacitances here? That would explain C4 and C10, which are impractically low for discrete components... \$\endgroup\$
    – Marcus Müller
    Commented Jan 23, 2021 at 20:16
  • \$\begingroup\$ Anyway, it's really the job of whoever builds or sells modules to specify what they do. So, vote to close as lacking necessary datasheets. \$\endgroup\$
    – Marcus Müller
    Commented Jan 23, 2021 at 20:16
  • \$\begingroup\$ Superregenerative receiver. not superheterodyne. They may have meant the average output, or DC component of the output, is 2.5V as these things tend to produce random 0V/5V pulses in the absence of good input signals. \$\endgroup\$
    – user16324
    Commented Jan 23, 2021 at 20:19
  • \$\begingroup\$ I absolutely agree with you on both of those points. Unfortunately I've not been able to find a datasheet or better schematic for this as it's a generic part made by a seemingly endless number of companies without an obvious original name brand. I'm not even sure of how to describe the circuit well enough to find a similar design with a schematic of a better grade. \$\endgroup\$
    – Jabermony
    Commented Jan 23, 2021 at 20:21

2 Answers 2

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the output circuit makes use of a duel opamp (LM 358) but my understanding is too flakey to derive the output voltage from this arrangement.

LM358 datasheet page 10:-

enter image description here

This tells you that maximum output voltage is Vcc - ~1.2 V = ~3.8V with 5 V supply. The ESP8266's rated maximum I/O input voltage is 3.6 V, however it probably has protection diodes that don't start conducting until pin voltage goes ~0.6 V above the power supply voltage, ie. 3.3 + 0.6 = 3.9 V.

So people who are using this combination without level shifting are just getting away with it if the power supply voltages are accurate. To be safe you should lower the output voltage by at least 0.5 V using eg. a voltage divider made from two resistors.

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Junction C5 and C6 connects to the base of transistor Q2 who does double duty as oscillator and audio stage.

That would explain C4 and C10, which are impractically low for discrete components.. really? lol I have .5pf and 1 pf in my junk box... 2pf (2.2pf) is a very common value used in UHF and microwave circuitry.

John

enter image description here

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