Scouts Home Made Telephone
Here is a project that we started along with the the Strowger Project which you will hear more about later in the additional supporting pages.
If you can image the scenario, you are together with you friends in a field and your Scout master has banished you to your separate tents for the evening but boys will be boys and you want to talk to your friends in another tent without your Scout Master knowing of course. Armed with this device you can make a call to the next tent for a little more than pocket change and some soldering expertise and your Scout Master will be scratching his head and none the wiser. This device has many perhaps unrealised applications this is just one of them but with upto date easy to obtain components that we hoped would add an element of fun.
- Identify what the purpose of the project is
- Obtain the Circuit Diagram
- Source components identified as that being required
- Build the Circuit
- Test the Circuit in the Lab
- Identify optimal components to improve performance
- Demonstrate the device on small unsuspecting aliens (kids)
- Field test the unit
Scope of Telephony Device
We intended to repeat a demonstration by Alexander Graham Bell’s, an experiment to transmit a signal that can be heard through a chain of people. The idea here was to align or daisy chain a number of school children together just by holding hands together, connect either end of the children to the device and watch them burn. Not really, we only used a small 9Volt PP3 Battery on each device so it was completely safe.
We can’t tell you where we obtained this circuit from we don’t know, it was an American Scout web site but despite repeated searches we seem to have lost it, if you can help us identify the original source of this document we are happy to accredit the original author. We have re documented the circuit as the original did not scale well and was difficult to read what the components where. This we hope has sufficient documentation, the design is a very simple one it uses the microphone as a speaker and a speaker as a microphone, sourcing the right component for this was quite challenging.
We managed to source all of the components bar one which is the Mic/Speaker from an electronic shop in Southend, Maplin Electronics however we are sure that you own local Electronic’s shop should stock all the components required. The Mic / Speaker was taken from an old analogue telephone that uses a high resistivity speaker in the hand set.
Vero board with copper strips along its length to solder to seemed the obvious choice and we very nearly got it right, it can be easy to follow but sometimes you forget to cut the odd track and the unit fails to function as expected. Use your own judgment on this what ever you find best, however as people have to handle the device or the wires attached to it, we found the joints and wires to joint kept breaking use plugin connectors where possible as this reduces strain on the wires. Affix the battery to the board you are mounting to by means of pclips as these weigh more than the board and components a rigid fixture is essential.
741 OpAMP’s are in general are very robust and unlikely to fail if you have a problem it is not likely to be there, make sure of your connections and ensure one of the paired devices is in listen mode whilst the other is in talk mode, tap the speaker with you finger nail on talk device whilst listening for the response on the other device. If they still don’t work try various high resistance speakers replacing the ones you have fitted.
The only component we needed to change was the speaker microphone the 16 Ohm devices from Maplin could not be made to work on this device so we opted to install old style carbon speakers to function both as a speaker and microphone.
Children are by far the best testing appliance you have at your disposal and often for a sweet or ice cream will happily comply with your request electrocute them with a measly 9 Volt battery. Seriously though if your devices survive the punishment inflicted by meaningful children it is more like to survive your field test.
We are in the middle of a new project as yet untested that with the use of a carrier wave will improve the performance and distance of this versatile telephone this will be tested at Chalkwell Park on Saturday 27th September 2008. Come along if you are able we believe this event and others are free as is the entry.