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What transistor to use?

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Unruly Radish

New Member
Hello all! I know very little about rewiring electronics so I'm sure this is a very simple (stupid) question.

I work on a search and rescue team and I'm trying to install a leading light on a search drone.

My problem is this; the wireless receiver works on a 1.5v (35 mA) AG3 battery which I need to use to switch on an 9 volt LED light powered by two 9v batteries.

I'm told a transistor would do this, but which one? Is there an easy way to figure it out?

Thank you in advance for your help!
 
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crutschow

Well-Known Member
Most Helpful Member
Not enough info.
What is the output signal from the receiver to turn on the LED?
How much current does the LED take?
 

Unruly Radish

New Member
Not enough info.
What is the output signal from the receiver to turn on the LED?
How much current does the LED take?
For the receiver I have no idea, nor the tools to measure it. All I know is that it is powered by a 1.5 volt battery across a small circuit board.

The LED draws 9-10A.

Many thanks!
 

spec

Well-Known Member
Most Helpful Member
Hi UR,

Welcome to ETO.

I see you are from the USA, so getting electronic components would not be a problem. Which state are you at?

Sounds like an interesting project.

A power MOSFET (a form of transistor) would be most suitable to switch your drone LED leading light on and off.

There are two polarities of MOSFETs: NMOSFETs and PMOSFETs. For example, if you used an NMOSFET (most common type) you would need zero volts between its gate terminal and source terminal to turn it off and around 6V to turn it on, and thus illuminate the leading light on the drone. MOSFETs only require a voltage to turn them on; they do not need any current (in practice this is not quite true for various practical reasons).

You would need to select a MOSFET that would handle the voltage and current involved. There are many suitable MOSFETS and we can recommend one. The MOSFET would cost around $6.00US.

But, as has been implied in the previous posts, we would need to know the characteristics of the signal from the radio receiver in order design a circuit that would turn the MOSFET on and off. My impression, though, is that a simple operational amplifier, or comparator circuit would do the job.

spec

Here is a data sheet for a typical NMOSFET that would do your job:
 

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Unruly Radish

New Member
Hello Spec,
Thanks so much for the in-depth and informative reply! I'm from California.

I will have to get a meter to measure the current coming from the receiver. Is that the last piece of the puzzle or am I just getting my feet wet?

So the NMOSFET in the data sheet will work as a switch for my application? I'm completely new to any of this so I apologize in advance for repetitive or foolish questions (I should probably educate myself on this a tad more before bothering you guys).
 

spec

Well-Known Member
Most Helpful Member
Thanks so much for the in-depth and informative reply!
No problems UR.:)
I'm from California.
:cool: If you enter that by 'Location' on your ETO user page it will show in the box at the left of your posts. Not only do some of us like to know where members are from, but it also helps with answers: component access, mains voltage, etc.
I will have to get a meter to measure the current coming from the receiver.
More or less, but we would like to know the voltage rather than current: connect a voltmeter between 0V (probably negative terminal of the radio 1.5V battery) and the output of the radio receiver. If you could let us know:
(1) The output voltage from the radio receiver in the 'off' state.
(2) The output voltage from the radio receiver in the 'on' state.
Is that the last piece of the puzzle or am I just getting my feet wet?
:) There will be a few more things to sort out, but with your cooperation we can almost certainly evolve a suitable circuit for you. How do you feel about constructing a circuit though?
So the NMOSFET in the data sheet will work as a switch for my application?
Absolutely correct. With zero volts on the NMOSFET gate, the resistance between the drain and source will be very high (like a switch turned off) and with around 6V on the gate the resistance between the drain and source will be zero (essentially), like a switch turned on.
I'm completely new to any of this so I apologize in advance for repetitive or foolish questions (I should probably educate myself on this a tad more before bothering you guys).
:joyful: Don't worry about that- you would hardly be asking for assistance if you were an electronics guru!

I have knocked-out the outline schematic below (based on assumptions) to illustrate the approach that I have in mind (pretty standard stuff) and I hope between us we can thrash out a detailed circuit to do your job. Most probably other members will also submit circuits/ideas so you may have a choice of approaches.:)

spec

Issue 02 of 2016_10_22

2016_10_22_Iss2_ETO_DRONE_LEADING_LIGHT_OUTLINE.png
 
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ChrisP58

Well-Known Member
You've said that the LED needs 9-10 amps, and you plan to run it off of two 9Volt batteries. If you're talking about the little rectangular 9V radio batteries you may have a problem.

Those little batteries just can't deliver that much current. That much current will pull their voltage down to the minimum on voltage of the LED. The LED will probably turn on, but won't have anywhere hear the brightness that you're probably hoping for.

Or am I misinterpreting something? Have you tested the LED with the batteries that you plan to use? How bright was it, and how long does the LED last?
 

Unruly Radish

New Member
More or less, but we would like to know the voltage rather than current: connect a voltmeter between 0V (probably negative terminal of the radio 1.5V battery) and the output of the radio receiver. If you could let us know:
(1) The output voltage from the radio receiver in the 'off' state.
(2) The output voltage from the radio receiver in the 'on' state.
I will find that out as soon as possible. Many thanks!

:) There will be a few more things to sort out, but with your cooperation we can almost certainly evolve a suitable circuit for you. How do you feel about constructing a circuit though?
I'm no stranger to soldering and regular household wiring. But as you can probably tell I'm definitely no electronics wizard like yourself.

How hard is constructing a circuit for the first time?

I have knocked-out the outline schematic below (based on assumptions) to illustrate the approach that I have in mind (pretty standard stuff) and I hope between us we can thrash out a detailed circuit to do your job.
Wow that's incredible, thank you! I feel like I owe you money or something. Definitely got my homework cut out for me.
 

Les Jones

Well-Known Member
Most Helpful Member
Hi spec,
I think it is unlikely that the output signal from a receiver powered from 1.5 volts will be more than 1.5 volts. I think it will need a bipolar transistor to interface between the receiver and the mosfet gate.
Unruly Radish,
I think you need to find and give more information on the receiver and LED light. If the receiver consumes 35 ma then it will not work for long on an SG3 cell. What is the voltage rating of the LED ? (You say you plan to power it from from two 9 volt batteries but you did not say if these are to be connected in series or parallel.) This must be a big drone to carry batteries capable of supplying 10 amps for a reasonable time,

Les.
 

Unruly Radish

New Member
You've said that the LED needs 9-10 amps, and you plan to run it off of two 9Volt batteries. If you're talking about the little rectangular 9V radio batteries you may have a problem.

Those little batteries just can't deliver that much current. That much current will pull their voltage down to the minimum on voltage of the LED. The LED will probably turn on, but won't have anywhere hear the brightness that you're probably hoping for.

Or am I misinterpreting something? Have you tested the LED with the batteries that you plan to use? How bright was it, and how long does the LED last?
We are using two parallel 9v transistor type batteries to power the IR LED. We have tested it and it works with our equipment, although as you say we don't get many flashes before the batteries are dead. But that's fine for our needs.

The company we bought it from explained that it was drawing the max current from the batteries, like a short circuit I'm guessing.
 

spec

Well-Known Member
Most Helpful Member
I think it is unlikely that the output signal from a receiver powered from 1.5 volts will be more than 1.5 volts. I think it will need a bipolar transistor to interface between the receiver and the mosfet gate.
Hi Les,

Maybe my post was misleading in some way, but I don't think I implied that the output from the radio receiver would be more than 1.5V; it might even be a current output.

For the radio/NMOSFET interface, I envisaged an LM393 comparator or perhaps LM358 opamp (both chips include OV in their input voltage range), running off the 9V LED supply. That will be low cost and give maximum flexibility of the input signal. It will also hit the NMOSFET gate with a nice big voltage.:cool:

spec
 
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Unruly Radish

New Member
Unruly Radish,
I think you need to find and give more information on the receiver and LED light. If the receiver consumes 35 ma then it will not work for long on an SG3 cell. What is the voltage rating of the LED ? (You say you plan to power it from from two 9 volt batteries but you did not say if these are to be connected in series or parallel.) This must be a big drone to carry batteries capable of supplying 10 amps for a reasonable time.
Hello Les Jones,

Yes I'm starting to realize that I didn't give you guys much to go on, sorry about that. However, thanks to you all I now know where to start looking.

I don't think the receiver draws 35mA (at least I hope not!), all I know about it at the moment is that it is powered by an AG3 battery that can put out 35 mA.

The LED didn't come with a lot of information as its military surplus, but the company we bought it from has it wired in parallel with standard 9v transistor type batteries for weight savings. It's only meant to "flash" on command for working in tandem with another drone.
 

spec

Well-Known Member
Most Helpful Member
We are using two parallel 9v transistor type batteries to power the IR LED. We have tested it and it works with our equipment, although as you say we don't get many flashes before the batteries are dead. But that's fine for our needs.

The company we bought it from explained that it was drawing the max current from the batteries, like a short circuit I'm guessing.
You haven't asked this, but it sounds like your battery arrangements would benefit from a re think.:D

spec
 

spec

Well-Known Member
Most Helpful Member
Probably! :p But those were the batteries they told us to use. :wideyed:
People say all sorts of things- I couldn't imagine a worse battery for that application.:arghh:

Can you tell us anything more about the LED: part number, voltage, you mentioned the current.

spec

UPDATE 2016_10_22

Three CR123 (non rechargeable) batteries in series would be far superior to two PP3 batteries in parallel. CR123 batteries each weigh 17 grams, giving a total weight of 51 grams compared to 90 grams for a pair of PP3 batteries

Alternatively three RCR123 (chargeable) batteries could be used.
 
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spec

Well-Known Member
Most Helpful Member
I'm no stranger to soldering and regular household wiring. How hard is constructing a circuit for the first time?
It will be a steep learning curve but, as you are logical, judging by your posts on ETO, and also your quick responses and helpful answers to our questions, I'm sure you can do it.:cool:

The circuit could be built on a small piece of strip board and would use fairly large components: through hole rather than surface mount.

If the components can't be obtained locally they can be obtained, mail order, from, DigiKey, Mouser, Element14 (Farnell), etc.

You will need:
(1) Fine soldering iron
(2) Fine lead tin solder (not lead-free)
(2) Small pliers
(3) Small wire cutters
(4) Screwdrivers
(5) Multimeter

(all of the above are available quite cheaply, in my town anyway)

And perhaps a few general tools, like files, drills, baby hack-saw, etc.

spec
 
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Unruly Radish

New Member
People say all sorts of things- I couldn't imagine a worse battery for that application.:arghh:

Can you tell us anything more about the LED: part number, voltage, you mentioned the current.

spec
You paint a dire picture :banghead:

I'll contact the company and get more information, its military surplus that's been "repackaged" so nothing printed on the side unfortunately.

Again thank you for your wealth of information and asking the questions that get me going in the right direction. You guys are life savers :)
 

Unruly Radish

New Member
Thanks Spec!

If you don't mind my asking, what do you do for a living?

Back to studying so I can understand everything you guys are telling me :sorry:
 

spec

Well-Known Member
Most Helpful Member
Thanks Spec!

If you don't mind my asking, what do you do for a living?

Back to studying so I can understand everything you guys are telling me :sorry:
No probs UR.

I don't do much for a living now, but was an electronics design engineer and, apart from some consultancy, retired. So no more meetings, reports, appraisals ..., just fun.:joyful: well mostly; I still have to mow the lawn.:arghh:

What do you do?

spec
 
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spec

Well-Known Member
Most Helpful Member
You paint a dire picture :banghead:
Apologies, but there is something out of kilter with the LED: a 9V, 10A LED= 90Watts, which would be onehellofa LED.

Also, as ChrisP58 implied in post #7, the maximum current a PP3 battery could supply is only around 200mA (0.2A) and then the terminal voltage would probably drop to around 6V. Also, PP3 batteries are troublesome little things.:eek:

My suspicion is that the LED may require a lower voltage and definitely take a lower current. Maybe you could make a couple more measurements to establish the LED characteristics if I explain what to do. Do you have a link to the supplier's web site?

spec
 
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