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I have a SRD-12VDC-SL-C relay with this configuration: https://www.iotwebplanet.com/product/relay-module/ And I want to trigger it with PIR Motion Sensor Detector Module HC-SR501 https://robu.in/product/pir-motion-sensor-detector-module-hc-sr501/ The problem with direct connection to PIR is that the trigger output of PIR is +3.3V however the relay needs low or (zero) voltage in order to switch. I do not see how the relay in the link above can be put into High-level trigger.

Relay module

Mohak Shukla
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    Isn't this basically your previous question? [Triggering 12v Relay with HC-SR501 PIR sensor](https://electronics.stackexchange.com/questions/517046/triggering-12v-relay-with-hc-sr501-pir-sensor) – Andy aka Aug 21 '20 at 11:13
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    Some relay modules have a built-in transistor to engage the relay when Vin = high. It is unclear how this module works and if it has that transistor. Also realize that the relay switch has 3 contacts, two of these go **open** when the relay is engaged. – Bimpelrekkie Aug 21 '20 at 11:14
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    Yes, it is similar to my previous question, but the problem statement and the solution I am seeking is different this time. – Mohak Shukla Aug 21 '20 at 11:26
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    If I see well, the relay module has an optocoupler input. A jumper selects the polarity we can control. – csabahu Aug 21 '20 at 11:26
  • As far as I can see, the relay module wants 5 V to switch not zero. And the PIR has a TTL 3.3V output. If there are pullups up to 12V you might fry your PIR. But the documentation of those modules in general is very bad. In general, if there's no real datasheet with exact values and sample wirings, you won't get very helpful hints here, because engineers prefer to work on exact data. – Ariser Aug 24 '20 at 11:45

1 Answers1

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Question

How to convert the following Low level trigger relay into High level trigger?

relay 1

Answer

Part A - Short Answer

No, you cannot convert this Low level trigger relay into High level trigger by capping the jumper to High, because there is no such jumper.

Part B - Long Answer

Relay features summary

B1. It is a Low level relay.

B2. It is not opto-isolated.

B3. It is not High/Low trigger jumper selectable.

Details explanation

B1. It is a Low level relay

(1) The relay module seems to have only one transistor, and that it all. For one transistor circuit, there are two possibilities,

(a) PNP BJT based level trigger,

(b) NPN based High level trigger.

pnp npn relay

relay module pic

The product sheet says it is Low triggerable, so that is it, no more and no less.

B2. It is not opto-isolated.

An opto-isolated relay module should have an optocoupler, which is almost always a four pin black plastic IC. The relay modules has only one three pin IC, which should be the PNP transistor. An example of a High level trigger, optoisolated relay circuit is something like below.

opto cct

optocoupler

B3. It is not High/Low trigger jumper selectable.

A High/Low level triggerable selectable by jumper of course should have a jumper. The picture and schematic is shown below.

H/L select relay

to continue, ...

References

(1) How to properly use a relay module with JD-VCC from Arduino/Raspberry? - tlfong01, EESE, 2020jun13

Appendices

Appendix A - Confusion of Relay and Relay Module

Newbies always mixed up relay and relay module. A relay is the small rectangular blue box. A relay module is the whole thing, consisting of the blue box relay and other components. The relay module almost always has no brand name.

relay switch 1

relay hystersis

/ to continue, ...

End of answer

tlfong01
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