BSA merit badge project

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MrDEB

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This is out of the Engineers mini note book I found while cleaning. Going to use for the Boy Scout electronics merit badge. Still need to layout on a breadboard but have printed circuit board designed and ready to print/etch for Thursday nite meeting.
This is a simple trivial circuit but thought I would share.
The LM741 schematic symbol is really not correct but dip trace didn't have a dip symbol for it so I used what I have.
I increased pad sizes and trace size. Made board large enough for easy assembly (12 year old scouts)
 
A lousy old 741 opamp will not work when its input (pin 3) is connected to its negative supply (pin 4). Pin 3 must be at least 4V more positive than pin 4. The circuit you coRpied uses TWO 9V batteries so that one produces +9V and the other produces -9V.

Your coRpy of the schematic is wired wrong and is missing a resistor at pin 2 of the LM741 opamp. A coRpy is a corrupted copy.

It looks like you are using a piezo transducer for lots of acoustical feedback howling. Then the LM386 power amplifier is not needed. The opamp can easily drive the piezo transducer.

Here is the your schematic and the original schematic:
 
I bread-boarded the schematic I posted using one battery but took out the .1uf cap going to ground. wouldn't work with cap between ground and battery (duh)
I have a hunch your right about a corrupt copy but its right out of the ENGINEERS MINI-NOTEBOOK Science Projects.
the speaker might be piezo? its a S125R-pc https://www.electro-tech-online.com/custompdfs/2012/01/70115878.pdf
don't really want to add another battery (would need more battery clips and batteries)
where did you find that schematic?
 
found a similar circuit

searched LIGHT LISTENER"
but no 741 op amp?
 

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Your speaker is not a piezo, it is 8 ohms with a coil and magnet.

The circuit I copied came from a Forrest Mims Mini-Notebook that was sold at Radioshack. It uses a 741 opamp with two 9V batteries. It drives an 8 ohm speaker which is impossible.

The Light Listener circuit you recently posted is almost the same as from the Mini-Notebook I posted but it uses an LM386 power amplifier instead of a 741 opamp.

Your latest schematic from DIY-Electronics projects is from India? Most circuits are simply copies from other websites. Many DO NOT WORK. It even has an LED project of mine and they did not ask for my permission.

The last two circuits you posted are missing a 10 ohm resistor and 0.05uF capacitor in series to ground at the output of the LM386 that are shown on every schematic in the datasheet that keep it from oscillating at a high frequency.

The circuit from India is missing a coupling capacitor from the output of the wrongly biased opamp to the volume control so IT WILL NOT WORK.
 
I tried with out the capacitor from pin 6 of the 741 to the pot and it does not work.
The original circuit I posted works but took out the capacitor from the battery - to ground and it works pretty good. kinda loud but then boys like things that are messy, dangerous and/or loud.
Wonder what ever happened to Forrest Mims the 3rd.
Somewhere I have the two original note books that Radio Shack sold. Both pretty similar but have different schematics.
Have you (Audio Guru) ever thought about doing a book?
 
Oh yea the 10 ohm resistor and .05cap.
Have lots of the resistors but not the .05 cap. Probably should include in circuit for proper data sheet protocol?
 
A 0.05uF capacitor was made 40 or 50 years ago. New ones are 47nF which is 0.047uF.
 
MrDeb and AudioGuru,

Looking at my old Forrest Mimms books and see the light wave receiver. Below the schematic we read:

Use to receive voice modulated voice waves and he also says OK to use single polarity power supply for non-voice reception.
Click to expand...

These circuits are much, much more amusing when they have a modulated light source to listen to. Forrest Mimms has several other interesting little circuits in those old but great books (well great in 1982 anyway). The bargraph light meter using 5 leds and photocell was amusing and the Audible Light Sensor using a pair of photo sensors to increase and decrease a tone. Something like that may be more interesting and visually appealing as a scout project.

Per AG, avoid those untested circuits that never seem to work from weird websites.

Just My Take
Ron
 
The first circuit I posted is an audible light listener. If you hole a tv remote in front of the photo-transistor you can hear the pulses from the remote.
kinda cool. We are building these come Thursday after I demonstrate how to solder.
I agree with some questionable web site schematics.
 
The first circuit has many things wrong as I showed. The 741 opamp does almost nothing and can be replaced by a piece of wire.
When its LM386 amplifier oscillates at a high frequency because it is missing some important parts then the battery will quickly be killed and the LM386 and battery might get hot.
Why are you letting the boy scouts make such garbage?

The "Light Listening Circuit" with only an LM386 power amplifier IC is much better because it can hear the clicks from a TV remote and can also hear voices and music if you have an opto modulator. The LM386 should have the 10 ohm resistor and series 0.05 (0.047uF) capacitor added to its output to ground as shown on its datasheet.
 
The first circuit works and it detects the tv remote signal as well. Am as adding the 10 ohm resistor and cap but going to try and remove the 741 from the circuit to see what I get
Reason for building this is its cheap, easy to read schematic and teaches how to solder.
the last project was a 7 mont button keyboard using a 555, 3 caps and a resistor ladder. Worked rather well but the boys had issues soldering the pushbuttons, getting resistors in wrong place as well as capacitors.
Will post on results of removing the 741 op amp.
 
The circuit with the old 741 opamp and LM386 has the LM386 with a gain of only 20 because a capacitor is missing between its pin 1 and its pin 8.
Removing the opamp and adding the capacitor will make it work much better with a gain of 200.

Do you use a photo-diode which is shown on your pcb and first schematic (both show it connected backwards) or do you use a photo-transistor?
 
using a vtt7122h
**broken link removed**
I basically connected the photo transistor directly to the lm386 but failed to add the cap.
it appears to work pretty good as is but will add the 10 ohm and cap to the lm386 output.
reason I used a diode symbol is that's what I had. will post revised schematic
 
revised schematic

here is where I am at.
seems to work pretty good on a bread board.
The photo transistor is a vtt7122h cheapest one they had
 
Your latest circuit still has the opamp connected completely wrong. I can't see how it can work like that.
Again I explain what is wrong and show one fix with a second battery or a second fix with biasing resistors instead.
 
we will try fix #2. I can't afford the second battery.
still have to buy clips.
THANKS again.
Now to redo the pcboard design. not to many changes.
 
You will probably find that some 741 opamps will not work from a 9V battery that drops to 6V during its life. It is 44 years old and was designed to use only a 30V supply. I would use an MC33171P that is low power and works when its supply drops to 3V. Its pin numbers are the same as a 741 opamp.

You have a 0.15uF capacitor feeding the 8 ohm speaker. Then the speaker will produce ultrasonic squeaks above 26kHz. Use at least 470uF for voices.
 
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