Remote Control Kill Switch_741 Op Amp

Thread Starter

Jsprite7660

Joined Jan 9, 2024
26
Hey All,

I am following directions from the link below to build a remote control kill switch using a pair of walkie talkies. It uses a 741 op amp to activate an automotive relay and the op amp is activated via the receiving walkie talkie.

Attached is a picture of my schematic and my problems are as follow;

- According to the directions, when the output of the receiver(going to pin #3 of Op Amp), goes above the reference voltage (pin #2) the output pin(#6) of the Op Amp goes "high." I have successfully calibrated the resistance so that the reference voltage at pin #2 is always .5 VDC. I have the receiver successfully sending 1.8VDC to pin #3 HOWEVER the output pin #6 only reads between 1.3 and 1.4 VDC.

- After that point the signal is supposed to go to a MOSFET transistor, I didn't have one so I put a BJT instead. But honestly I haven't read that far ahead because I'm not getting a "HIGH" signal from the output of the 741 op amp..........if the output of the 741 is less than the input coming from the receiver what's even the point of it???? Right? lol

Instead of using the op amp and all, can't I just build a basic amplifier circuit that would boost the 1.8VDC to 12VDC in order to activate the car relay? If this is the way to go, does anyone have any schematics I should refer to? Why isn't the op amp doing what it is said to do in the instructions? Any help would be greatly appreciated!

Thank you!

Link to the original online directions; https://www.instructables.com/Remote-Kill-Switch/#discuss

My Schematic.pngBreadboard.png
 
Last edited by a moderator:

dl324

Joined Mar 30, 2015
16,911
Welcome to AAC!
According to the directions, when the output of the receiver(going to pin #3 of Op Amp), goes above the reference voltage (pin #2) the output pin(#6) of the Op Amp goes "high." I have successfully calibrated the resistance so that the reference voltage at pin #2 is always .5 VDC. I have the receiver successfully sending 1.8VDC to pin #3 HOWEVER the output pin #6 only reads between 1.3 and 1.4 VDC.
Easier to read schematic:
My Schematic.jpg
When you use connection dots, you don't need humps for wire crossings, and we put pin numbers on the outside of the symbol (where you have the voltages).

You are using an inappropriate device.

LM741 don't work well from a single supply and your input voltages are lower than what the device guarantees. Also, you shouldn't use an opamp as a comparator. Comparators are faster. Most comparators are open collector (e.g. LM393), so you need a pull-up resistor.

And you need a current limiting resistor on the base of the transistor and I can't tell what polarity the transistor is.
 

schmitt trigger

Joined Jul 12, 2010
897

Thread Starter

Jsprite7660

Joined Jan 9, 2024
26
Welcome to AAC!

Easier to read schematic:
View attachment 312237
When you use connection dots, you don't need humps for wire crossings, and we put pin numbers on the outside of the symbol (where you have the voltages).

You are using an inappropriate device.

LM741 don't work well from a single supply and your input voltages are lower than what the device guarantees. Also, you shouldn't use an opamp as a comparator. Comparators are faster. Most comparators are open collector (e.g. LM393), so you need a pull-up resistor.

And you need a current limiting resistor on the base of the transistor and I can't tell what polarity the transistor is.
Hey thanks for the info, I'll look into all this
 

dl324

Joined Mar 30, 2015
16,911
I love when forums are for beginners like myself but then when you ask for the help people respond as tho you're supposed to know what you're doing.
The transistor doesn't like having more than about 0.7V across the base-emitter junction.

You wouldn't need a current limiting resistor with a MOSFET, but you'd use a low value resistor anyway to stop the opamp from oscillating because it's driving a capacative load.
 

Papabravo

Joined Feb 24, 2006
21,225
Welcome to AAC!

Easier to read schematic:
View attachment 312237
When you use connection dots, you don't need humps for wire crossings, and we put pin numbers on the outside of the symbol (where you have the voltages).

You are using an inappropriate device.

LM741 don't work well from a single supply and your input voltages are lower than what the device guarantees. Also, you shouldn't use an opamp as a comparator. Comparators are faster. Most comparators are open collector (e.g. LM393), so you need a pull-up resistor.

And you need a current limiting resistor on the base of the transistor and I can't tell what polarity the transistor is.
Where are the power connections for the 741? A 741 OpAmp damn skippy won't work without them. The way you learn about things you should know is to read the datasheet and pay attention to what it is telling you.
 

MisterBill2

Joined Jan 23, 2018
18,464
So far EVERY responder has been a beginner!! There are no power connections to the 741 op amp.
Note that I had started this comment before papabravo responded, addressing the same thing.
In addition, to avoid damaging the transistor, which is not drawn right. a resistor is needed to limit the base current.
Why didn't any of the first ten replies mention that???
 

Papabravo

Joined Feb 24, 2006
21,225
So far EVERY responder has been a beginner!! There are no power connections to the 741 op amp.
Note that I had started this comment before papabravo responded, addressing the same thing.
In addition, to avoid damaging the transistor, which is not drawn right. a resistor is needed to limit the base current.
Why didn't any of the first ten replies mention that???
Beats the pure hell outta me!
 

Thread Starter

Jsprite7660

Joined Jan 9, 2024
26
So far EVERY responder has been a beginner!! There are no power connections to the 741 op amp.
Note that I had started this comment before papabravo responded, addressing the same thing.
In addition, to avoid damaging the transistor, which is not drawn right. a resistor is needed to limit the base current.
Why didn't any of the first ten replies mention that???
Thank you for that! I will correct those mistakes.
 

Thread Starter

Jsprite7660

Joined Jan 9, 2024
26
The transistor doesn't like having more than about 0.7V across the base-emitter junction.

You wouldn't need a current limiting resistor with a MOSFET, but you'd use a low value resistor anyway to stop the opamp from oscillating because it's driving a capacative load.
Thank you
 
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