555 Based Drone Synthesizer With LFOs

Today I have an update to the 555 Based Drone Synthesizer which I posted last week. I was having a lot of fun with my Drone synth but wanted to expand on it to get a bit more variation from the sound. To do this I added a set of LFOs to modulate the frequency of the drone oscillators.

To accomplish this I built a second set of three 555 based oscillators. These new oscillators are identical to the ones in the drone synthesizer except for a change to the capacitor between pin 2 and ground. By increasing this capacitor from 0.01uf to 1uf I was able to lower the frequency of the square wave they produce. This makes them perfect for use as LFOs.

I then connected the output (pin 3) of each of these new oscillators to the control voltage input (pin 5) of the corresponding oscillator in the drone synthesizer through a 1K resistor. An additional modification that can be made to this circuit to provide further control would be to replace this 1K resistor with a 10K or 50K ohm potentiometer which would allow you to modify the amount which the LFO modulates the oscillator in the drone synth.

In this experiment I have used LFOs but you could easily control the drone synth in other ways as well by providing a control voltage to pin 5. I am interested to see how this set-up would react to a control from a sequencer or keyboard and may try this in the future.

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555 Based Piezo Trigger

I’ve always been drawn to drum pads and kits. They are lots of fun and offer a slightly more tactile method of control then rows of pots and switches. So today while I was playing around on my breadboard I was drawn to pull out some piezoelectric disks and start experimenting.  What I’ve come up with is a very simple drum trigger circuit that you can build and experiment with.

This circuit uses a 555 timer set up in monostable mode. A monostable 555 timer will output a square wave pulse whenever it receives a trigger pulse from the piezo disc at pin 2. The pulse output from the 555 can then be adjusted through the 500K ohm pot placed between V+ and pin 7. The output pulse is then sent into the base of a 2N3904 transistor which works as a gate between the audio source and the speaker. This means when the pulse from the 555 is high the audio will pass through the transistor and when the pulse ends and the 555 output goes low the transistor will block the audio from passing.

If you are interested in adjusting the pulse length beyond what is available using the pot this can be achieved by adjusting the electrolytic capacitor between pin 6 and ground. By lowering the value of this cap you can shorten the range of pulse lengths available. Conversely by increasing it you can access a longer range of pulses.

By setting up 4 or 5 of these piezo trigger circuits you could create a fairly versatile set of drum pads. Since the audio source can be switched out or developed further there’s a lot that you can do to expand on the acoustic possibilities of your drum kit. You can try experimenting with different oscillators, Filters, LFOs, White Noise Generators or anything you want.

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555 Oscillators in Series

I’ve been spending a lot of time lately playing around with 555 timer chips and wanted to quickly share my latest creation. This project uses four 555 timers each set up as a standard astable oscillator. I’ve then connected the output from pin 3 of each oscillator to the control voltage at pin 5 of the next subsequent chip. Essentially this means each 555 timer is working as an LFO for the next oscillator to it’s right. I also increased the size of the capacitor between pin 6 and ground of the two left-most oscillators in order to  lower their frequency.

I was quite pleased with the range and depth of sounds it produced however, it should be said that I built this as a proof of concept and it is not fully flushed out. I would be very interested to try a similar setup with a different waveform. I feel like this idea would really come into it’s own if used with a triangle or sine wave oscillator which produced a wider range of tones. I have also been experimenting, with some success, with adding capacitors between the output and control voltage inputs to smooth the square wave slightly and create a saw tooth pattern. Without an oscilloscope on hand however this is proving difficult to optimize.

This is also a circuit which can be easily expanded by adding additional oscillators and admittedly there is a little voice screaming in my head to take it to it’s logical conclusion. I expect in my near future I’ll spend a rainy afternoon stringing together as many 555 circuits as I can fit on my breadboards and see what I end up with. I’ll be sure to share the results.

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VTech Apple Part 5 – 555 Trigger Oscillator

circuit bending, loop trigger, 555

I have to admit for some time I have been stalled with my Vtech Alphabet Apple circuit bending project. I love the toy aesthetically and have always felt like there should be more bends available then what I was able to find. However even after hours of experimenting and dissecting this toy I was left feeling somewhat underwhelmed with the results. Over the weekend though I brought it back out determined to turn it into a more functional instrument, and to do that I needed to create a trigger oscillator.

If you want to get caught up before going forward don’t hesitate to visit my previous posts on this toy:

VTech Apple Part 1 – Kill Switch and Line Out

Vtech Apple Part 2 – Exploration and Pitch Adjustment

Vtech Apple Part 3 – Voltage Starve

Vtech Apple Part 4 – Body Contacts

Since I hadn’t had any luck finding a loop on the board I moved on to less straight forward methods. I decided what I needed was a way to send a signal at repeating intervals to one of the contacts on the button matrix to trick the Vtech Apple into thinking a button was being repeatedly pressed and trigger a repeating sound. By generating this signal independently I could manipulate its frequency and control to suit my needs.

Once I had a clear definition of what I needed to get the job done the solution seemed all to clear, what I needed was a 555 timer. By setting up an astable 555 timer as a trigger oscillator I could route the square wave signal into the button matrix to repeatedly trigger the button (or buttons) of my choosing. Further by using a potentiometer I would be able to adjust the frequency of the square wave and therefore control the time between button presses as needed.

555 astable
This is a simple mock up of the circuit I used. Note that for this to work the positive voltage must be supplied by the toy itself and the ground must be share with the toy as well. This is easily accomplished by running the power from either the positive power connection on the Apple’s circuit board or directly from the kill switch installed in Part 1 . Just ensure it is connected at or past the kill switch so that the kill switch will remove power from the oscillator as well. Similarly the ground can be connected to any ground point on the circuit.

Astable 555 circuit
After testing my plan using a breadboard I put together this small 555 timer circuit on a scrap piece of perf board I had laying around from a previous project. I did my best to keep everything as small and compact as possible as my space inside the toy is somewhat limited. I’ve seen some circuit benders using what is called the “Dead Bug Method” in these situations to further minimize the size of the circuit. When building a dead bug circuit the components and connections are soldered directly to the pins on the IC rather than onto a piece of perf board. This can be an excellent way to shrink the circuit for those really tight fits but also leaves you with a more fragile product so since I could get away with using a board in this toy I did in order to get more stability and durability from the circuit. I will revisit dead bug circuits in a future post but in the interim there are many demonstrations of the method on YouTube if you deem it necessary.

Wiring 555 Oscillator
Once I had my 555  trigger oscillator circuit built the next step was to install it in my Vtech. I started out by planning positions and drilling holes for the potentiometer, switch and LED. Once these were in place I began the process of wiring the leads I had left on the 555 trigger oscillator circuit to their respective locations on the toy. On the diagram below I have marked the approximate paths of each wire upon installation. In planning this mod I did my best to limit the number of wires crossing between the back and front sections of the toy as these wires tend to get put under a lot of stress when the toy is being opened and closed. To achieve this I pulled power from my kill switch and sent ground directly to the negative terminal on the battery box. Since the switch and pot are mounted on the back portion this means only the pulse out wires have to cross over to the front half of the apple.

Wiring Vtech Apple

You may also notice that there are two pulse out wires leading from the switch to the button matrix. I used a 3 position on-off-on switch for this bend which allowed me to send the pulse to two locations based on the switch position (as well as nowhere in the off position) By connecting the opposing sides of the switch to different positions on the key matrix I am able to choose between two different buttons when running the oscillator. If you were so inclined and had the space to work with you could take this even further using a rotary switch or patch bay to allow you to select where the pulse was being sent. If there is a specific key you are after which you are not finding by touching the pulse to the solder points on the matrix try using your probe to connect sets of two points on the matrix together. If you find that this is necessary to get the input you desire this can be done by bridging the two points with a transistor and feeding the pulse into the base (more on that in a future article).

Finished Vtech Apple Loop
Once everything was wired I secured the 555 circuit and LED with hot glue, taped down all the loose wires and closed up the toy. I have to say after playing with it a bit I am really enjoying this modification. I’ve been able to produce a host of strange noises and effects and without having to use a hand to continually press the buttons I feel like I’m finally able to take full advantage of the other bends on this device. I’ve been having particular fun using the power starve to produce glitches in conjunction with the continuous oscillating noise produced by raising the rate of the 555 to high frequencies.

That’s all for today but I hope you guys have fun. Happy soldering!

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