# Help amplifying a tiny current

#### Avondale

Sep 4, 2016
19
Hi ElectronicsPoint

I have a temperature sensor module and i want to use it to activate a relay
the 3v coin battery on the temp sensor works great, the output turns on when the selected temperature is reached
when the 3v coin battery is used on the relay works great and feeds through 70mA roughly
when i join both together there is only 10mA going through the relay from the temp sensor
the relay wont fire at this amount and i need to figure out how to amplify it
i am a beginner at components but i have tried a transistor and got it up to 50mA but it refuses to go higher

I have attempted to draw out the circuit, please advise : D

Last edited:

Moderator
Jan 21, 2010
25,510

#### Avondale

Sep 4, 2016
19
thanks @(*steve*)
i have a question about what VCC would represent and what the unmarked input would represent

#### Avondale

Sep 4, 2016
19
i have drawn up a representation of it!
check original post

#### Alec_t

Jul 7, 2015
3,677
is there a tool you can link me to so i can show my circuit in an easy way for a beginner like me?
The schematic editor in LTspice (free download from Linear Technology) is easy to use. You can also simulate the circuit operation (though learning how to do that well is a bit daunting).

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510

It would be useful to get some information about the "temperature switch". The circuit you've drawn *may* be ok, but since it doesn't work there's more we need to know.

#### Avondale

Sep 4, 2016
19
sure!
(missing the actual temp sensor on the end)

works great, outputs 3v when temp resistance is more than adjusted resistance (i believe that's how it works)

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
ok, the next step is to connect the battery to the relay and read the voltage across the battery when it is connected.

#### Avondale

Sep 4, 2016
19
ok, the next step is to connect the battery to the relay and read the voltage across the battery when it is connected.
2.12v

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
2.12V is really low.

can you try the circuit you have using a stiffer 3V supply, say 2 x AA batteries?

#### Avondale

Sep 4, 2016
19
2.12V is really low.

can you try the circuit you have using a stiffer 3V supply, say 2 x AA batteries?
that helps! 3v

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Is it behaving better?

#### Avondale

Sep 4, 2016
19
i'm trying to get a reading on the mA coming out of the transistor now but there's too many wires to hold together lol

#### Avondale

Sep 4, 2016
19
now i'm getting 1.5v through the relay from the transistor

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Do you want the relay energized when the led on that module is on or off?

#### Avondale

Sep 4, 2016
19
it seems to go off when the temperature goes above setting, which is when i would like the relay to activate

#### Avondale

Sep 4, 2016
19
i guess things are not as simple as i previously thought them to be : p

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
The LM393 is open collector logic. This means it can't source current. Because of this your base drive current will be totally dependent upon the value of the pullup resistor connected to "DO". Since the output LED lights when 393's output pin goes low (not high) I would think that you should be using a PNP transistor with its Emitter connected to Vcc and the relay coil connected between the collector and GND. You will still need a series connected base resistor to limit base current.

Chris

#### Alec_t

Jul 7, 2015
3,677
Relays are current-hungry devices. Could you use a FET instead for switching a load?

Last edited:

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Here's a schematic of what I described in my last post. I do have a question that i don't think anyone asked you. What's the operating Voltage and Current rating of your relay. Relays under 5V are not common.

Chris

Replies
18
Views
2K
T
Replies
3
Views
1K
Daniel
D
Replies
4
Views
70
Replies
4
Views
122
Replies
7
Views
128