I am very new to the world of electronics and have run in to an issue with a relay. I have a Wemos chip that has GPIO pins. I measured 3v coming from a pin but it doesn't seem to trigger the 3v relay. My limited knowledge tells me this is a current issue. Is it possible to increase the current with an additional component?

I'd be greatful for any advice.


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    \$\begingroup\$ Can you provide a schematic as well as the relay part number and/or a link to the relays datasheet? \$\endgroup\$ Jan 18 '18 at 21:52
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    \$\begingroup\$ What pin? What relay? You expect us to guess?? \$\endgroup\$
    – Bort
    Jan 18 '18 at 22:04
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    \$\begingroup\$ It takes a minimum of 120mA to drive that coil... \$\endgroup\$
    – Ron Beyer
    Jan 18 '18 at 22:31
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    \$\begingroup\$ @Kyle - You could use an NPN BJT (2N3904 is a standard part) or N-channel MOSFET (2N7000 is a standard part). \$\endgroup\$
    – vofa
    Jan 18 '18 at 22:54
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    \$\begingroup\$ For what it's worth, I have my doubts that a 2N7000 N-MOSFET will work here. Looking at the 2N7000's data sheet, for VGS=3V, it looks to me like ID<100mA for VDS>0.5V. If you want to use an N-MOSFET to drive the relay's coil, you might try instead something like a Microchip Technology (formerly Supertex) TN0702N3-G. \$\endgroup\$ Jan 19 '18 at 4:41

According to the srd-03vdc-sl-l relay datasheet, the 3-V version of coil needs 120 - 150 mA of current to operate. This coil is 25 Ohms only. No general-purpose microcontroller can output 120 mA at 3 V. It is likely that the measurement was done in some wrong way.

The Wemos board is based on ESP-8266EX IC, which has 12 mA load capability on GPIOS. From VIH (75% of Vcc) at nominal 12 mA it follows that the output stage has about 70 Ohm of internal resistance. Loading a 70-Ohm GPIO with 25 Ohms should result in maybe 1 V on the coil. This won't work. The design needs a transistor switch to operate this relay.


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