control a relay with alarm clock ...?

[alex4421]

hi,

i want to control a relay wot max capacity of 24 v and around 2 A
mostley a relay needs about 12v to switch.
i messuered the volts on the buzzer of the alarm clock and the max i've got was about 0,2 v ... what do i need to do to let it control a little relay ?

 

[RODALCO]

Alex, try to disconnect the buzzer and fit a 1k or 2k2 ohm resistor across the terminals.

See what the voltage is then. If more than 0.5 or 0.6 volts you can drive a NPN transistor and drive your relay.

 

[alex4421]

i only have a 1000 kohm ? the rest is around 330 ohm...

 

[RODALCO]

Put 3 x 330 ohms in series which gives you 1 k. ohm.

I picked that value as a guess, it is not that cirtical but it is better that the driver to the buzzer has some loading on it in the form of a resistor.

 

[alex4421]

i tryed with the 1000 kohm resistor and the max i've messeured was 1,3 volts !!

 

[RODALCO]

1.3 volts is enough to turn a transistor on, with.

The 12 volt supply for the relay needs to be taken from a plugpack supply.

 

[alex4421]

can you explain it a little bit better pls
or maybe draw it ?

you mean that the resistor is a switch that would contol the power of a 12 v supply to switch the relay ? can i use the same 12 v source as the source thats being contoled by the relay ? so 12volts source to control relay and the same source for being controled ?

 

[alex4421]

i have looked aroud in my little box with elektronic stuff and found two transistors(i think)
they are both the same and have a bit of text on them :
"c547c
W37"

can i do something with these ?

 

[RODALCO]

Something along the lines of this diagramme you need to build.

The clock buzzer or speaker you may have to disconnect to have enough voltage to drive the BC 547.

Use a 12 Volt dc plugpack to drive the relay.

 

[alex4421]

so i have to connect the two wires from my speaker to the rest so , the schedule above but without the buzzer , can i use that ^12 v source at the same time for something else ?

 

[Norlin]

that transistor sounds like a BC547C NPN transistor
https://www.alldatasheet.com/datasheet-pdf/pdf/50733/FAIRCHILD/BC547C.html

 

[alex4421]

here is a page with relay's on , it's from a shop in belgium so most is dutch but the things you need to now areenglish ...

**broken link removed**
so i need to buy :

- 12v power source
- diode :1n914
- small relay less then 100 ma for coil
- resistor 1kohm an a 100 ohm ?...
- capacitor 10 µf
- transistor bc547 (witch i already have xD)

and that's about it ?

and if i understand it well , the transistor opens the circuit of the 12 v accu and the 12 v will close the relay ? so the transistor works like a switch ?

 

[RODALCO]

You are lucky because I'm Dutch and can read it, I emigrated to New Zealand 20 years ago.

Get the REL26 which can switch 7 Ampères at 230 Volts.

Your clock speaker may be supplied with ac so a small capacitor may be required before the 100 ohm resistor. A bit of trial and error may be required here.

The BC 547 is a standard NPN transistor and act like a switch. When about 0.6 volts is applied to its base it will turn on and the relay will pull in and closes the contacts. The 100 ohm resistor is there to limit the base current.

Be sure that the diode is the correct way around, otherwise you blow up your transistor.
If the relay draws more than 100 mA use a 2N2222 transistor which can handle a bit more current.

These transistors are cheap so a couple of spares would'n do any harm.

Powersupply: get a plugpack 12 volts dc output 300 mA , 500 mA or 1 Amp.
These can be used for other projects as well.

Good luck with your project.

Raymond

 

[alex4421]

i have a accu thats 12 v and 1Ah ...

 

[RODALCO]

You can use the 12 volts accu as your power supply.

As long the relay coil is 12 volts it should work ok.