This project offers students a cost-effective way to build an AM tuner that is easy to incorporate into a variety of design situations. The Printed Circuit Board (PCB) can be either purchased ready made, or students can manufacture their own from the mask provided in the notes.
Suitable for Key Stages 3 & 4 (ages 11 to 16)
Based on the TA-7642, an updated version of the popular MK484 IC
Needs only a few external components to build a high quality AM radio
They contain both detailed teacher and student sections, providing research information, and structured tasks
The notes explain in detail how radio signals are transmitted and received
They also look at how the design of radios has been influenced by style and technology of the day, from the first radios of the 1900's, through the introduction of the transistor, right up to todays digital radios
Students are offered a variety of starting design briefs and guidance on how to develop their ideas
Detailed diagrams and instructions support the radio circuit construction, and the notes provide suggestions for possible case designs and methods of manufacture
The teacher section of the notes includes suggested lesson plans and mapping to the National Curriculum and a range of homework assignments
The component kits are supplied in multiples of five and include:
TA-7642 and transistors
Ferrite rods and wire
Tuning capacitors and switches
Battery holders and jack sockets
Resistors and capacitors
Screws, nuts and washers
Pre-drilled PCBs are supplied separately in packs of 5, order code 70-0120
Schools have the choice to either purchase a PCB or manufacture their own from the mask provided in the notes
Q. Question by Andy (June 2013) was answered suggesting a 32R earpiece. But the 'you may be interested' items has a crystal earpiece of 2MR. Before ordering a class worth of kits, please can you confirm which is the preferred earpiece.
A. Hi Mark, thank you for your question. Either of the following can be used, it is more your preference as both will work: single ear piece 35-0165, twin ear piece 35-1207.
Q. Which of your low-cost speakers would be appropriate (multiple school project)?
A. Hi Andy. Thank you for your question. The kit is designed to use a 32 ohm standard set of headphones and as such the output stage is not suitable to power a speaker.
Q. Please could you suggest a low-cost ear-bud set for use with this kit?
Are there ready-made plastic boxes that might serve as a housing?
A. Hi Andy. Thank you for your question. We have the following ear buds: 35-0165 or 35-1207. A suitable small box could be something like the following: 30-3360 or 30-6230.
Q. what control knobs do I need to fit the tuning capacitor please ?
A. Hi Colin. Thank you for your question. We no longer have the wheel that fits the tuning capacitor However 37-0261 could be modified to fit without much difficulty.
Reviews
Excellent beginner's radio project
Reviewed by: Marcus Jones - Thursday, December 21, 2017
The kit uses the TA7642 TRF radio IC, a clone of the once widely used ZN414. It is ideal for beginners enabling working radios to be made with a minimum of components. I have a working radio based on this circuit that I use round the house. This kit is very good value.
One problem often encountered is that the TA7642 is very sensitive and can become unstable, especially with new batteries or strong signals. Many circuits include a 5k to 10k variable resistor between the 1k load resistor and +ve supply rail to allow the gain to be varied. This also works as a crude volume control. I would recommend this is done. It also allows one of the two 1k resistors to be dropped (I use a 470R).
The circuit can be simplified by using direct coupling between the TA7642 and BC548, removing the coupling capacitor and 100k bias resistor. The emitter resistor is reduced to 100R to maintain Ic.
The website below provides a lot of information on this IC and shows circuits with these modifications:
http://members.iinet.net.au/~cool386/zn414/zn414.html
Strictly speaking the moving vanes/ spindle of the variable capacitor should be connected to the earthy side of the radio not the input, which is what happens wiht the current PCB layout. However, by drilling an additional hole the VC can be shifted along, moving the centre connection to the junction of the 100k and 10n capacitor, and putting both gangs in parallel, which also increases the tuning range.
I hope this ideas are helpful to anyone wanting to experiment with this kit.
