# Toy Organ

Discussion in 'The Projects Forum' started by edwardholmes91, Apr 9, 2013.

1. ### edwardholmes91 Thread Starter Member

Feb 25, 2013
200
20
Hello,

During the summer holidays I will be working at a summer camp in PA, USA teaching simple electronics to children between the ages of 7 and 15. I have been asked to come prepared with a selection of different circuit that I could teach the children and that they could build.

In search of circuits I came across Forrest M. Mims series of Engineer's Mini Notebooks and decided to buy them. I have to say they are an interesting read so far and there seems to be lots of fairly simple circuits that would be suitable to teach to children.

The "Toy Organ" on page 23 of Volume I struck me as a fun circuit to teach the children, without requiring too much theory. For those of you without access to this book I have copied the circuit onto the computer:

So my questions are... it surely must be possible to instead of using capacitors that seem to be in the E6 range use resistors? If my understanding is correct (I'm scratching my head here, because we're going back a number of years since I studied this sort of stuff) the 555 timer uses an RC setup to provide it's time base? Therefore, a range of resistors could be used and a fixed capacitor value instead?

Also thinking about fabrication, using tactile switches would be an expensive solution. Therefore it got me thinking about touch sensitive switches. Correct me if I'm wrong... but a Darlington pair could be used to create a touch switch, which would operate when you touch your fingers on two PCB pads?

The circuit suggests 15 capacitor values, but you could just as easily use more or less. From memory I believe a Darlington driver IC package is available in both 7 and 8 pair versions? Using two would give you 14 buttons and in effect two octaves?

This website looked quite useful because it gives the different frequencies of notes on instruments. As can be seen, the toy organ more than covers a lot of the range. The human ear can only hear 20Hz to 20kHz I believe, so the range isn't a problem.

I believe that touching more than one switch at once would give interesting results... but this isn't a problem, I'm not looking for a polyphonic solution or anything fancy. Just a fun circuit to teach kids, whilst explaining the theory behind it.

Looking forward to hearing from people, if there is anything else that I can provide let me know and I will do my best

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2. ### MrChips Moderator

Oct 2, 2009
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The toy organ is a great idea. However, using capacitors in order to arrive at the correct tones would be a royal pain. There is a larger selection of precision resistor available than there are capacitors.

I would opt for a single capacitor and use the push buttons to select the resistors.
For extra trouble and expense, you can use trimpots instead of fixed resistors. This would allow you to adjust each note independently.

3. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,329
The administrator over at Electro-Tech-Online published a circuit for a toy organ, doing exactly what you describe. I would NEVER choose to change the capacitor values. As MrChips mentioned, there is a much larger selection of resistors out there. Here's the link: http://www.electro-tech-online.com/content/568-toy-organ.html

Hope this helps.

Regards,
Matt

4. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
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I did that once, but if you connect them in the way shown in the ETO link, you have to adjust the first one first, since all the others depend on it. However, if you can figure out how to have each pot set up individually, it would make it much easier.

Dec 12, 2012
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6. ### elec_mech Senior Member

Nov 12, 2008
1,513
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I can't speak toward touch buttons, but Tracecom suggested a cracklebox in another post I read today and that uses touch pads and generates tones. Video here, plans and schematics here. Looks interesting.

ULN2004 (7 outputs) and ULN2804 (8 outputs) - both will work with 9VDC.

Another thought is using paper clips and nails or screws as contacts to act as switches.

Not sure how many projects you'll be working on, but I was part of college club that taught basic electricity principles to education majors. By far the most well-received project we did was making simple motors. The most common one I found is often referred to as Beakman's motor (taught on a science show I think).

We bought 12 packs of AA's from the Dollar Tree, 1/2in magnets from craftstores or Walmart, and I got the magnet wire from work from scrap. Instead of paper clips, I found attaching #2 safety pins to the AA battery with a rubber band saved a lot of time and hassle. The pins were found at Hancock Fabrics - I mention this because I went to several stores and most only sell packs of mixed-sized pins. I think any true fabric store will carry these. This is a cheap, easy to make project. Even those who had difficulty kept working at it when they went to other stations.

Another simple motor is the homo polar motor. While it appears simplier, it's a little harder to balance and get going. We did not have much interest in this one at all.

I don't know how complex you're going to go, but a 555 monostable timer might be another fun one to make. The kids could make their own timers for races, baking, etc.

absf likes this.
7. ### DerStrom8 Well-Known Member

Feb 20, 2011
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Avoid talkingelectronics whenever possible. The guy who runs it doesn't know a thing about electronics, and often makes stupid mistakes in his circuits. If you use a circuit from talkingelectronics, I HIGHLY RECOMMEND checking it against other circuits first. I, personally, just don't trust the site at all.

8. ### RayInMS Member

Dec 12, 2012
89
1
Good info - thanks.

9. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
I have recently got a collection of old elektor magazines dating back to 1975. And I think I have come by some circuits for toy organ. However I think the classic light dimmer circuit. Connect to the mains would ideal for that age segment of age (Err... just joking)

10. ### absf Senior Member

Dec 29, 2010
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Nevertheless, you can use the site to get project ideas on 555 projects. It is amazing what the 555 can do.

A 555 metal detector or a 3-player "who press the button first" game show timer using 555 might be good projects too.

Allen

11. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
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Sure, it has a bunch of project ideas, but I still recommend checking them against similar circuits elsewhere to make sure you don't destroy something

edwardholmes91 likes this.
12. ### edwardholmes91 Thread Starter Member

Feb 25, 2013
200
20
Haha, thanks. I'm keeping well away from mains with children. I'm thinking AA battery packs and PP3 batteries

Some people have posted some really interesting ideas. I think the best thing to do would be for me to get one of these built up on breadboard and have a play around with them. I've got some 555 timers knocking about somewhere and I'm sure I can find an old 8R PC speaker somewhere.

13. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,329
If you use an 8 ohm speaker with the 555, don't forget the coupling capacitor. You might also want to use a transistor to drive the speaker better.

EDIT: You may not actually need the coupling capacitor, since you're only working with a square wave. Not sure though....

14. ### t06afre AAC Fanatic!

May 11, 2009
5,939
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Here is some projects that are easy to build. They use quite generic parts. The transistors in the siren should be easy to replace with other compatible ones. They are not exotic in any way

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• ###### cricket simulator.pdf
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absf likes this.
15. ### GopherT AAC Fanatic!

Nov 23, 2012
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How many kids will you be doing this with each session and per summer?

What budget will you have per work station and per kid (if any).

16. ### edwardholmes91 Thread Starter Member

Feb 25, 2013
200
20
I can't really answer any of those questions I'm afraid. I have been hired to teach children aged between 7 and 15, electronics. They will be split into rough age groups and will choose from a range of projects or make their own. The number of children depends on how many are interested in the activity or not. I have just been asked to bring ideas and possible circuits along with me.

Last edited: Apr 13, 2013
17. ### GopherT AAC Fanatic!

Nov 23, 2012
6,564
4,461
Try to teach them something that they see move. As someone else said and I can confirm, you don't have to do amazing things to get a kid interested. Sometimes, the really simple things get them interested and the complex things are so abstract that they just fall into a daze.

Show them how to connect a battery to a light bulb, a good old fashion bulb that works in either direction. Then give them some motors and LEDs where you already soldered a small resistor to one lead so they don't damage it.

Add a switch. Add a more complicated switch. Make them wire a switch where a bulb is on in one switch position and a motor turns in the other position.

I have a box full of 3 volt bulbs, let me know where to send them when the time nears. I am sure other members can send a few feet of wire, some old toy motors, and switches or battery boxes or even a wire cutters or strippers if you need them.

Your getting thrown into a tough situation with a tough audience and it sounds like you were not given a lot of information to allow you to make a plan. Good luck and ask the team for help, I think you will be amazed how many people would send some of their simple junk box items your way (or to your US summer camp).

18. ### edwardholmes91 Thread Starter Member

Feb 25, 2013
200
20
Thank you for all of the helpful advice I really appreciate it. From what I can make out they have projects already available or the children can design their own. The electronics section on their website gives a little more information: Camp Watonka Science. I believe there will be others that have attended the camp before as staff who will also be teaching it, so I should have a hand from them. I was just asked to come armed with a few fun, but also simple circuits that they could build. Until I get there I don't fully know how complete their labs are, but from what they make it seems fairly good.

The Toy Organ is coming along nicely. I have put it onto breadboard and also calculated the resistor values for two octaves. It seems to operate quite nicely. I made a bit of a stupid mistake by thinking that I could use a darlington driver array for making lots of touch switches... but I forgot that a darlington driver is inverting. Unless I use two transistors for each key which would make a more complex than I'd have hoped circuit I don't think it's going to be possible.

So far I have just been using a flying lead and touching it on the resistor legs, so kind of like a Stylophone. I think this is going to be the best way of going forward because it makes the circuit really simple and easy to explain.

Once I have got some proper designs together I will post a few attachments including circuit diagram and spreadsheet that I used to calculate values.

19. ### edwardholmes91 Thread Starter Member

Feb 25, 2013
200
20
Right guys, I have been doing a bit of designing and come up with this schematic. Most of it came from this website. Please see attached schematic as a PDF file.

There will be a whole string of resistors wired in series, but I couldn't easily put them on the schematic and it's irrelevant to the design. Rather than using switches I will be using a stylus like the "Stylophone" toy available in the 80's I believe (it was before my time I'm afraid... although I have seen one and had a play before!)

A couple of questions really... is the LED and series resistor over the supply ok to provide a power indicator? I will be using a 56-0445 from Rapid Electronics. This will reliably operate on anything between 2mA and 30mA. To save power I will be operating it as close to 2mA as possible, using a 3K3 resistor will give a current of 2.136mA.

The stylus will be a flying lead and a sort of pen, used to touch the keys. I considered making something you could put on your finger... but this meant that the keys needed to be bigger and I wanted to keep the width of the board below 20cm.

The 10uF capacitor value was taken from the website I linked to above, however reading a post on here it seems I probably need a bigger value capacitor. I have tested the circuit using the 10uF and not had any problems though. Using the calculation:

$C=\frac{1}{2.\pi.F.Z}$

...with a frequency range of 260Hz to 1.05kHz and an impedance of 8R I get 76.5uF so would a 100uF be suitable instead of 10uF as in the schematic?

Also I noticed this post which refers to adding a volume control. I have a 50k logarithmic potentiometer which I was thinking of using... can anyone tell me if it is a suitable value and if it is wired correctly on the schematic?

With regard to the values for the different notes, I took the different frequencies of notes from an interesting Wikipedia page. I decided that two octaves with sharps and flats seemed like a reasonable number of notes (And could also be reduced for a smaller board if required). VR2 allows the adjustment of the first and indeed any other note thereafter... kind of like a tuning control I suppose. Using a little Excel knowledge, a few calculations and extensive use of the Goal Seek feature I was able to calculate the appropriate resistor values for each note. Of course these wouldn't be preferred values and although a variable resistor could be used for each value... this would make the circuit more complex than I require. A square wave and a crude PC loudspeaker is hardly going to accurately replicate the notes of a piano! I therefore decided to choose the closest preferred values from the E12 series. You will see in the speadsheet a column called "R2 E12 Value". All columns with a green heading are assuming E12 values and a preset for tuning the first note. From this frequency deviation in both Hz and percent from the original note have been calculated.

So anyway, enough of my ramblings... I have been working on a PCB artwork and also formulating ideas for a casing made using laser cut clear perspex. I'm not really prepared to post a copy of the artwork just yet until I have finalized the schematic and also fully checked it.

Maybe this little project once finished could feature in the Finished Projects section of the forum? If indeed there isn't already one there?

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20. ### edwardholmes91 Thread Starter Member

Feb 25, 2013
200
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Has anyone got any comments about the other schematic I posted last? Any corrections? I'm keen to know if the power indicator is okay? If the capacitor before the LS1 should be 100uF instead of 10uF. And also if a 50k log pot is suitable for the volume and if I've wired it correctly? Other than those few things I think it's ready for me to start working out a PCB for.

Looking forward to hearing from everyone