Wireless frequency control for 555 timer

Thread Starter

corasan

Joined Jul 18, 2023
32
What could be a better way to wirelessly control the frequency of a 555 timer with a potentiometer as an input? I thought of using a varactor diode to change the frequency by adjusting bias voltages on the varactor's cathode. Also, can you recommend 4-bit ADC and DAC IC models for this design? My idea is to wire the potentiometer to an ADC to serve as the digital input of the HT12E module, then transmit it via a 315MHz RF module. The decoder then receives the signal, then the DAC converts it into the bias voltage of the varactor. This is my first time building a wireless circuit, so I might be missing some important concepts. I wanted to ensure the feasibility of the idea before buying the components, so please suggest if there's anything I can improve.

Here are my key components:
Potentiometer
ADC
HT12E (encoder)
315MHz RF module
HT12D (decoder)
DAC
Varactor
555
 

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Sensacell

Joined Jun 19, 2012
3,475
Hmmm. Probably simpler to build a voltage-to-frequency converter- or directly synthesize the signal using a microcontroller.
A varactor is only going to change a few pF in capacitance, (look at the datasheet)

If you must... try a digital pot or a Vactrol (LED and CDS cell) as a variable resistor instead.
 

BobTPH

Joined Jun 5, 2013
9,130
Your parts list:
Potentiometer
ADC
HT12E (encoder)
315MHz RF module
HT12D (decoder)
DAC
Varactor
555
Parts list with microcontroller:

Potentiometer
315MHz RF modules
2 x Microcontroller

Plus, much more flexible and accurate, and easy to add additional functionality.
 
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Thread Starter

corasan

Joined Jul 18, 2023
32
Your parts list:

Parts list with microcontroller:

Potentiometer
315MHz RF modules
2 x Microcontroller

Plus, much more flexible and accurate, and easy to add additional functionality.
I am well aware of how this circuit may be simplified using microcontrollers. However, I am trying to design the circuit using ICs only.
 

Thread Starter

corasan

Joined Jul 18, 2023
32
Connect the four data output pins from the HT12D to four binary weighted resistors to replace R1 in the standard 555 astable circuit.
Like this?
1711209082770.png
I assume these binary weighted resistors would act as the DAC itself. Does the 4T12D output 5V at 1111? What should be my R2 value?
 

crutschow

Joined Mar 14, 2008
34,680
Like this?
Close.
The 4T12D does output 5V but you would need to add diodes in series with each output, otherwise the resistor is shorted to ground when the output is low.
Alternately you could have the outputs drive a small P-MOSFET or a CD4066 quad-switch as a high-side switch to the resistors.
How about using the weighted resistors to pin 5 to control the frequency? Will that be possible?
That would not give an accurate change of frequency with input.
 

crutschow

Joined Mar 14, 2008
34,680
Below is the LTspice sim of an example circuit using a CD4066 quad analog switch IC to switch the resistors that controls the 555 astable frequency:
In this case it sequentially shorts out the selected resistor(s) in a binary sequence.

1712073075254.png
 
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Thread Starter

corasan

Joined Jul 18, 2023
32
Below is the LTspice sim of an example circuit using a CD4066 quad analog switch IC to switch the resistors that controls the 555 astable frequency:
This seems pretty cool! I have never thought of this. The CD4066 shorts out the resistors for each binary HIGH to reduce the total resistance of the network. Do I assign the lsb to the least value resistor, and the msb to the highest value resistor?

How about the transmitter side? What commercial IC can I use to convert the pot values to a 4-bit binary for the 4T12E input data?
 

Thread Starter

corasan

Joined Jul 18, 2023
32
Alternately you could have the outputs drive a small P-MOSFET
This uses the same idea as the CD4066 right? Sequentially shorting each resistor. What P-MOS specifications should I be looking for?
you would need to add diodes in series with each output
I feel like this would be more compact than the two alternatives (P-MOS and CD4066). If I add diodes in series, how will the diode voltage drop affect the frequency response of the circuit? My idea is that the diode-resistor network would add up the binary voltage, supplying the THR pin with a voltage to set the frequency.
 
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DickCappels

Joined Aug 21, 2008
10,226
Please add a bypass capacitor between the power supply and ground for the NE555. I assume the RF modules have their own bypass capacitors.
 

Thread Starter

corasan

Joined Jul 18, 2023
32
Please add a bypass capacitor between the power supply and ground for the NE555. I assume the RF modules have their own bypass capacitors.
So far, I only know the capacitor values for digital ICs. Would there be a different value for analog ICs and RF modules?

Here's a list of my planned bypass capacitor values:
100uF - supply rail
0.1uF - CD4066, HT12E/D
10uF & 0.1uF - NE555
? - 315MHz RF module
 
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AnalogKid

Joined Aug 1, 2013
11,128
Unless I missed it, I don't see the frequency range you want to achieve with the 555. What is it.

Also, what adjustment resolution do you need? IOW, What is the smallest step-change in frequency when the digital value changes by 1 LSB?

ak
 

Thread Starter

corasan

Joined Jul 18, 2023
32
I don't see the frequency range you want to achieve with the 555
Right now, I don't have any intentions to specify the frequency response of the circuit, since I'm just doing it for practice. All I want to make now is do wireless controls using IC. Maybe I can come back to this design later whenever time comes! I assume the bandwidth and resolution would just depend on my resistor and capacitor values, which I can easily adjust when needed.
 
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