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I am new to electronics and I am following the book "Make: Electronics" by Charles Platt.

I have built a circuit with a 12V DC power supply, a 1K resistor, a relay, two LEDs, and a momentary-on switch.

enter image description here

When I turn on the power supply, one of the LEDs lights up. However, when I press the switch down (and hold it down) that LED goes out but the other one does not light up at all.

I have made sure all connections are good, and I have replaced every single component in the circuit, but it just doesn't seem to be working.

The only thing I can think of is that I was mis-sold a relay by Maplin. I asked for a "DPDT non-latching 12V DC relay" as it says in the book. The one I have has "1A 125V AC" and "2A 24V DC" written on it.

Any suggestions? Cheers.

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  • \$\begingroup\$ Check again the polarity of the non-working LED. \$\endgroup\$ – Cornelius May 17 '14 at 15:40
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    \$\begingroup\$ Do you hear the relay 'clicking' when you press the button? What happens when you swap the LEDs? \$\endgroup\$ – Wouter van Ooijen May 17 '14 at 15:45
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    \$\begingroup\$ @Andyaka Hooking up an LED backwards should not harm it, as long as the series resistor is in place. There are even bi-color LEDs made up of a red LED and a green LED in the same package connected head to tail, where only one lights depending on the polarity applied. \$\endgroup\$ – tcrosley May 17 '14 at 16:03
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    \$\begingroup\$ @tcrosley Not true dude. Most LEDs I've seen have an absolute maximum reverse voltage of about 5 volts and putting one the wrong way round (even via a resistor) on a 12 volt supply will likely damage it. \$\endgroup\$ – Andy aka May 17 '14 at 16:18
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    \$\begingroup\$ Try rewiring it as per my answer dude. Not all relays are wired the same. Pins 4 and 13 are poles of the changeover contacts not pins 6 and 11. \$\endgroup\$ – Andy aka May 17 '14 at 16:35
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You've wired it up incorrectly. See this : -

enter image description here

Try swapping the wires to the two most central pins. I got this PCB detail from here and although it is not the same part it is classified as a BT type 47. The Maplin link doesn't show the wiring so hopefully this one will be compatible.

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  • \$\begingroup\$ If I swap the LEDs on to pins 6 and 11, leaving the switch on pin 8 and the power on pins 1 and 16, I get the same problem unfortunately. \$\endgroup\$ – User 17670 May 17 '14 at 16:41
  • \$\begingroup\$ Read what I said, the two middle pins you are using need to be swapped i.e. swap pin 4 wiring to pin 6 and the former pin 6 wiring to pin 4. Also, look at the circuit diagram on the relay picture - pin 4 is where power in goes to the switch and clearly pin 6 is one LED output and pin 8 is the other. Contacts 9, 11 and 13 are not required for this experiment. \$\endgroup\$ – Andy aka May 17 '14 at 16:44
  • \$\begingroup\$ BTW, is it normal that relays (or components in general) do not come with wiring diagrams? Is it normal that I should expect a 'BT Type 47' style, or would I just be expected to mess around and find out for myself? IDK what the expectations are in the electronics world \$\endgroup\$ – User 17670 May 17 '14 at 16:51
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    \$\begingroup\$ As a golden rule (and never broken AFAIK in years) I never buy a component that doesn't have a data sheet (.pdf file) - what the suppliers may put on their sales site may easily be mistaken or lack certain info - I always read the manufacturer's data sheet even on the most basic of components and I bet that all the regular guys on here will 100% say the same without exception. \$\endgroup\$ – Andy aka May 17 '14 at 16:55
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    \$\begingroup\$ +1. I'm near my EE degree and the only thing I learned is: read the datasheet. \$\endgroup\$ – Vladimir Cravero May 17 '14 at 17:23
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  1. Try to use Multimeter in continuity mode and check the contact and the pole if the multimeter is beeping. 2. Add a free wheeling diode at the relay coil like this .enter image description here hope it helps
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  • \$\begingroup\$ Should I make just one continuity check, and should I do it where you have drawn the diode? \$\endgroup\$ – User 17670 May 17 '14 at 16:24
  • \$\begingroup\$ at the contacts , pole , NC , NO. Place one probe at pole and other at NC or NO and trigger the relay to check it. \$\endgroup\$ – Adi May 17 '14 at 16:38

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