TRF Radios 4, 3 Transistor BC548 Radio

1: THE MEDIUM WAVE MINI
Adapted from an article by A.J. Crighton published in the August 1978 edition of Everyday Electronics Magazine

EVERYDAY AND PRACTICAL ELECTRONICS website

After the introduction of the Ferranti ZN414 Integrated Circuit in the 1970's there were many articles published that provide details of simple radio circuits that used the I.C. The ZN414 is no longer made, but a new and almost identical I.C., the MK484, can substituted in all the ZN414 circuits and has proved to be a least, if not more, effective.

In the 1970's the ZN414 could be quite expensive to buy compared to cheap transistors and so this article was published in Everyday Electronics magazine as an inexpensive alternative. Today some readers may find it difficult to obtain the MK484 so this circuit still holds great value.

This design uses three cheap transistors, the BC548 as can be seen from the diagram below.

The Medium Wave Mini circuit diagram

The tuned circuit, L1/C2 selects the required station, transistor TR1 providing the r.f amplification. The signal is then passed to TR2, a small amount of the signal is fed back from the collector to the base of TR1 via the tuned circuit. A mixture of both radio frequency and audio frequency circulates in this feedback loop, the r.f. reinforcing that coming via the tuned circuit, while the a.f. is passed to the audio stage.

Transistor TR3 provides audio amplification that is enough to drive a crystal earphone which is plugged into the socket SK1

Most of the components are mounted on a small piece of plain matrix board having 13 by 7 holes as shown below:

Wiring details for the matrix board and other components

Modification required to the open frame jack socket
to make an 'on-off' switching arrangement, and
other layout details

Connections to the components are made using single cored wire on the underside of the board, this layout enables a smaller layout than using stripboard, however some experimenters may still wish to alter the layout to suit construction on 'strip board'.

The aerial coil L1 is 80 turns of 32 s.w.g. enamelled copper wire close wound onto a small ferrite rod. The jack socket serves as both a connection for the earphone and a power switch. One of the switch contacts must be bent slightly out so that they 'make' contact when the phone plug is inserted rather than the usual 'breaking' action when the plug is inserted. As can be seen from the circuit diagram the power switching is applied in the negative side of the circuit.

The tuning capacitor C2 is a typical miniature polyvaricon type that is widely available from electronic component suppliers and which can be seen in most pocket type transistor radios. If this variable capacitor has two gangs then usually only one gang will be required although some types may require that the two gangs are wired in parallel in order to be able to tune to the lower end of the medium wave band.

PARTS REQUIRED
Resistors - all 1/4 Watt carbon +/- 10%
R1	100 k
R2	2.7 k (see text)
R3	100 k
R4	1.5 k
Capacitors
C1	0.01 uF Ceramic
C2	250pF Variable
C3	0.1 uF Polyester
C4	0.47 uF Electrolytic
Semiconductors
TR1, 2, 3	BC548 npn silicon
Other Items
SK1	3.5mm open frame jack socket (see text)
L1	45mm x 10mm Ferrite Rod and 32 s.w.g. enamelled copper wire (see text)
B1	1.5 volt battery
0.1 inch Matrix Board 13 x 7 holes; Small plastic Case 60 x 40 mm; Crystal Earpiece; Tuning Dial or Knob for C2; Thin Hook-Up Wire

The completed circuit board

USING THE MEDIUM WAVE MINI RADIO

Before switching on for the first time, double check that all the components are wired up properly and that the battery is connected the the correct way round. When you are happy that there are no mistakes plug in the earphone, this will energise the circuit and some background noise should be heard in the earphone.

Adjusting the tuning control should produce a few local transmitters, the number of stations available will depend on the location and the strength of signals available. The tuning is quite sharp and therefore some care is needed when using the control. The ferrite rod is directional and so the radio should be rotated in both planes for maximum signal pick-up for each radio station.

If a whistle occurs then this can be reduced by increasing the value of the 2.7k resistor, R2, to 3.3k Ohms. It may be possible to replace this fixed value resistor R2 with a sub-miniature preset resistor to make fine adjustments.

The Completed MEDIUM WAVE MINI Radio

Above is a MINI-RADIO made by Karen Orton.
Visit Karen's excellent technical webpage HERE
http://www.techlib.com/Karen.htm#Matchbox%20Radios

Also see this page by Charle Wenzel:
http://www.techlib.com/electronics/reflex.htm#match

LINKS:

Visit the EVERYDAY AND PRACTICAL ELECTRONICS Website

PAGES WITH ARTICLES BY RADIO EXPERT FELIX SCERRI:

High Quality AM Receivers:

http://www.crystalradio.net/scerri/index.shtml

http://www.crystalradio.net/

http://www.xtal-sets.com/

Hi Fi AM Receiver Designs:

http://sound.westhost.com/articles/am-radio.htm

OTHER ARTICLES

ESP by Rod Elliot - "Mad as Hell" & lead free solder directive:

http://sound.westhost.com/madashell.htm

ESP by Rod Elliot - CFL's (Compact Fluorescent Lamps)
http://sound.westhost.com/articles/incandescent.htm