Small relay and transistor circuit trigger at <1v

[Roff]

You might find a transistor that can stand 30 mA base current.
Or find a germanium transistor.

Another way may be to shunt the silicon b-e junction with a silicon diode and a small series resistor.
This diode will steal some current from the base at Vbe > 0.5v and you can adjust the diode series resistor to have the base current close to the abs. max. rating.

Any common transistor can handle 30mA base current. Audioguru and I suggested adding 47Ω in series, mostly for insurance. The diode is unnecessary.

 

[Edwin2]

No one has suggested that he connect the relay contact to the left side of the base resistor.

Yes, it was suggested @ 10 August, by b_reagle.
Watch his diagramme....

Edwin

 

[Roff]

Yes, it was suggested @ 10 August, by b_reagle.
Watch his diagramme....

Edwin

So it was. My bad.

 

[archana87]

u can use TIP3IC transistor and 2.2kohm resistor..connect 2.2k to base.emitter ground..collector to common of relay.but tell me do u want to drive a motor???in fwd and reverse motion???

 

[audioguru]

I would like to see the 1V vibrator motor go into orbit when the 12V is connected to it as a latch for the relay.

 

[Roff]

u can use TIP3IC transistor and 2.2kohm resistor..connect 2.2k to base.emitter ground..collector to common of relay.but tell me do u want to drive a motor???in fwd and reverse motion???

Have you read the entire post? The OP says he has 1 volt available to drive the transistor. 2.2k will not provide enough base current to reliably saturate a TIP31C, or any other transistor, when driving a small relay

 

[Jaw174]

yeaaa

soo nothin, eh?

i found a $10 hobby kit that has a relay switch on when it gets dark out. i took out the optical transistor thingy and connect it to the cell phone. takes a bit of time, but it works. just have to work on the sensativity. takes a good 3 rings/ vibrates on the phone to get the relay to latch.


so, im still interested in a smaller more straight-forward circuit.

 

[pc88]

How about using a comparator (like a LM339) to detect the 1V signal? The comparator can drive a transistor which in turn drives the relay.

 

[Jaw174]

ok, that looks promising. i dont know how an op-amp or comparitor works tho...

i looked up the application chart - perhaps someone can suggest a circuit?

https://www.electro-tech-online.com/custompdfs/2008/10/AN-74.pdf

Page 6 caught my eye - Limit Comparator with Lamp
Driver.**broken link removed**

do you think that would work?

how would you vary the sensativity? i would imagine you ned to provide a reference - perhaps by a variable resistor tied to Vcc?

 

[pc88]

how would you vary the sensativity? i would imagine you ned to provide a reference - perhaps by a variable resistor tied to Vcc?

In the above schematic, the lamp is lit if V_IN is between V_A and V_B which may be set by fixed resistors or by pots.

If you only need to test for V_IN < V_A, you can eliminate the bottom comparator. Similarly, you can eliminate the top comparator if you only need to test for V_IN > V_B.

 

[jeandeau]

How about using a decade counter chip and an op amp. Raise the 1 volt signal to 4V with the first stage of the op amp(powered by the 5V). I assume this would be sufficient voltage to signal the counter input. Connect the second count position to the base of the transistor or mosfet and the third count position oof the counter to reset. The counter could be configured for up to 8 loads toggled sequentially. Does it work?

 

[SinjaNinja]

Same problem here

There has to be a work around to this, I am trying to trigger a 12v relay with an alarm clock, the current is pulsed at 1v intervals. I can't believe this is such a big problem.
Does the MOSFET thing work or not? Someone please weigh in on this.


Thank You

 

[OldTechie]

Ok, so far, all I've seen is a lot of guessing about what will work for the OP. Let's do a bit of math to figure it all out.

The OP says that he has 1 volt at 30 ma available when he wants the relay to turn on. Let's just look at a typical low power transistor datasheet. Let's start with the venerable 2N2222A. It's available from many distributors such as Mouser and Digikey.

The datasheet says that it can handle 600ma of collector cuurrent. Typical 5V relays of this nature have a coil resistance of 160 - 200 ohms. So, the collector current the transistor will have to handle is roughly 30ma or less. Not a problem for our transistor.

Now. we need to figure out how much current the base needs in order to switch the transistor into saturation. Typical Vbe for a silicon transistor at turnon will be about 0.65 volts. So, if we have 1V to drive it with, we know that we have adequate voltage to turn the base-emitter junction on (1V > 0.65V).

Now, how much current will we need into the base? From the datasheet, we see that with a collector current of about 30ma, the transistor has a DC current gain (Hfe) of about 100. That means that if we need a collector current of 30ma, the base current will have to be .03ma/100 = .0003ma, or 300ua. The base-emitter voltage is 0.65, so the voltage that the resistor needs to drop is 1-0.65 = 0.35V. So, the base limiting resistor will need to be approx 0.35V/0.3ma, or 1.2K. The 100K resistor is there to prevent collector-base leakage current from turning the transistor on.

Now, we know that the 1V input can turn our transistor on with enough gain to energize the relay. The only thing remaining is to latch the relay. See the diagram below for that. The OP will have to get a double-pole relay in order to latch it on. Otherwise, we would have to build a bistable latch (flip-flop) in order for it to work. Simpler to get the appropriate relay.

View attachment 50091

Cheers,
Dave M

 

[OldTechie]

Oops... I forgot to mention that in order to release the relay, you need a NC switch (pushbutton?) in order to break the current in the relay coil circuit.

Sorry 'bout that, Chief!

Dave M

 

[audioguru]

Now, how much current will we need into the base? From the datasheet, we see that with a collector current of about 30ma, the transistor has a DC current gain (Hfe) of about 100. That means that if we need a collector current of 30ma, the base current will have to be .03ma/100 = .0003ma, or 300ua.

Absolutely not.
The datasheet for the 2N2222A shows that its hFE is 100 only when it is not saturated with a collector voltage of 10V. But as a switch the transistor must saturate with a collector voltage of 0.1V or 0.2V. When the transistor is saturated the datasheet says that the base current must be 1/10th the collector current.

 

[Bvs]

Hi,

I am also having the same problem has anyone figured out a solution here. please reply if u get it.

 

[dougy83]

Hi,

I am also having the same problem has anyone figured out a solution here. please reply if u get it.

You can use an SCR, e.g. C106D https://www.onsemi.com/pub_link/Collateral/C106-D.PDF
It will trigger and latch on if you feed ~20uA/0.6V into its gate.