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Grandparents revenge toy

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going to visit my daughter in Vancover Washington in September and need a project to build (for 4 year old grandson) as I get up at 4am each morning so need something to do.
Was going to build a more complex Theremin but this small unit will work just fine.
**broken link removed**
adding a LM386 amp and etching a circuit board
just curious about the amp section = looks right but ??
Audioguru would have a field day with this circuit.
I want to implement some day a PIC using ADC, maybe several PICs for a simulation of a 5 string guitar using photodiodes on the neck for the different tones.
just kidding about grandparent revenge but it sounds good.


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Hi MrDeb,

before you design a PCB for the circuit please review it thoroughly.

This is what I saw on the first glance:

U4 is wired the wrong direction.

Last edited:
You should also:
Capacitively couple the "audio" to the top of the volume control (pot3). You might also want to add a resistor in series there because the LM386 doesn't need that much drive.
You could also probably get away with using only one 4069 since a passive LPF would be OK considering the gain available in the LM386.
I realize my mistake on U4

I just inserted as is without checking polarity = thanks
The circuit is posted from
**broken link removed**
but it dosn't have an amp.
Will add a cap if you feel it needs one.
Say a .1uf and a 1k resistor? (looking at the Forrest Mimms circuit and it shows the cap and resistor.
forgot to add the antenna at the R1/R2/C3 junction
the second 4069 controls the pitch null calibration
rechecked the data sheet

pin 3 is the input for a TO-92 case
the data sheet dosn't show the pins? but looking at the website I obtained the circuit from it has pin 3 connected to the positive side of the battery.
the part is LP2950ACZ-5.0/NOPB
I just used a T)220 (7805) componet in the schematic. I should of doubled checked before posting. the regulator is connected correctly using the TO-92 case.
Thanks for the input
boy are tantalum caps expensive

added a cap and resistor as per web schematic
**broken link removed**
added the antenna
question is the cap C7 connected to the speaker
just an electrolic or other?
the data sheet shows a 220uf I think.
the voltage reg is correct its a TO-92 case
the schematic shows a 27K resistor in parellel with the output jack but instead of using a jack for headphones I am using an LM386/ 8ohm speaker. Why the 27K resistor = because of the headphones??
any other suggestions??


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Hi again,

you might want to change the gain of the amplifier.

Here is a suggestion for three gain settings.



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Question on parts

C5 = ??value
C3 = 47nf?
input cap = tant or ceramic. why so big?? 100nf??
where did you find the schematics? if data sheet then I need to re-review
C5 should be 10µF

C4 = 100nF for a lower -3dB point of 160Hz, 47nF will give -3dB @ 340Hz.

What's the lowest frequency it needs to pass?

Last time I checked a 100nF capacitor costs the same as a 47nF capacitor.
Most of the 3rd CD4069 is not needed.
You don't need more gain than the basic gain of 20 in the LM386.
I would make it like this:


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A real Theremin has a volume antenna.

Here are some basig designs.
**broken link removed**
**broken link removed**

I have an idea of how you could simplify the volume a little. I'll post one it if you're interested.
Lets take a look at a simplified volume control

On **broken link removed** the website it has a Cds volume control that sounds like it could be fun (grandson is 4 years old).
will remove the second 4060 as per audioguru. hope it dosn't mess up the operation of the unit.
in the directions it mentions using a ground plane as there is high impedances involved.designing a 2x board with top layer a ground plane.
Found another mistake

I had antenna connected to power.
Removed the extra 4066 and changed the 10k pot to a 100k pot.


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I can't count and I goofed.
There is no 3rd CD4069. The original circuit has two of them and the circuit must be like that. One of them has its supply voltage trimmed with the "pitch null" pot.

I think the simple project will sound like a variable frequency continuous tone buzzer.
It would sound better if you could gate the tone on and off.
Link how this unit sounds

This link will give you a sample how this minimum Theremin sounds.
sounds weird and am sure my grandson will love it.
hopefully removing the "extra" 4069 will not affect the sound.
still thinking about how to integrate a PIC using ADC or DAC, throw in a VCO then add 5x the circuit for a 5 melody instrument using photo diodes instead of an antenna.
BIG circuit mistake

after drawing revised version with removing the second 4069 I realized that I have to have two 4069's as the pot2 controls the supply voltage to U2 which is adjustable via pot2.
U1 doesn't have a variable supply
it was a nice idea but won't work. nice try and thanks Audioguru
where did you find the schematics? if data sheet then I need to re-review

You won't find that schematic in the datasheet. Just combine all three possibilities of gain settings using jumpers.
here is revised schematic

Have to use two 4069's
wonder if I could add second unused parts of 4069's as a second theremin but change some values for a "second" voice or octave?


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Make two more oscillators (like the original lower ones) with the unused inverters. Set their frequencies a little different from the original. Then mix the outputs with diodes near the input to the amplifier and the sounds will be chords with 3 sounds playing different frequencies at the same time.
will input into simulation prg

like Tina or Multisim to see what frequencies the oscillator is presently outputting then put different values for second oscillator.
instruction say to have the first set of 4069's (antenna input) as far away from oscillator as possible so I may need a larger board?
planning on a top and bottom layer ground planes on the pc board w/.30 clearance around all traces (worked pretty good the last boards I etched.
present design is 2.9"x4.1"


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