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This circuit was originally designed to power a motorcycle intercom from the vehicle supply system. This type of intercom, which is used for communication between driver and passenger, generally requires quite a bit of power. In order to improve intelligibility there is often elaborate filtering and a compander is sometimes used as well. The disadvantage is that a battery doesn’t last very long. You could use rechargeable batteries, of course, but that is often rather laborious. It seems much more obvious to use the motorcycle power supply instead. 

A 9-V converter for such an application has to meet a few special requirements. For one, it has to prevent interference from, for example, the ignition system reaching the attached circuit. It is also preferable that the entire circuit fits in the 9-V battery compartment. This circuit meets these requirements quite successfully and the design has nonetheless remained fairly simple. In the schematic we can recognise a filter, followed by a voltage regulator and a voltage indicator. D1, which protects the circuit against reverse polarity, is followed by an LC and an RC filter (C3/L1/L2/C1/R1/C2). This filter excludes various disturbances from the motorcycle power system. Moreover, the design with the 78L08 and D3 ensures that the voltage regulator is operating in the linear region. The nominal sys-tem voltage of 14 V can some-times sag to about 12 V when heavy loads such as the lights are switched on. 

Although the circuit is obviously suitable for all kinds of applications, we would like to mention that it has been extensively tested on a Yamaha TRX850. These tests show that the converter functions very well and that the interference suppression is excellent. Link

Most engineers will recognise the problem: Your circuit needs a stable 1 or 2 MHz clock generator (in the author’s case it was for a Pong game using an old AY3-8500). A suitable crystal is not to hand so you cobble together an RC oscillator (there are plenty of circuits for such a design). Now it turns out that you don’t have exactly the right capacitor so a preset pot is add e d to allow some adjustment . Before you know it the clock circuit is taking up more space on the board than you had hoped. 

Providing the application does not demand a precise clock source a tiny 8-pin microcontroller may offer a better solution to the problem. It needs no additional external components and an old ATtiny15 can be found quite cheaply. Another advantage of the solution is that clock frequency adjustment does not involve changing external components and is not subject to component tolerances. 

The microcontroller’s internal RC oscillator is already accurately calibrated to 1.6 MHz. With its inbuilt PLL, internal Timer 1 can achieve up to 25.6 MHz [2]. By configuring internal dividers the timer can output a frequency in range of roughly 50 kHz up to 12 MHz from an output pin. The difference between calculated and the actual output frequency increases at higher frequencies. A meaningful upper limit of about 2 MHz is a practical value and even at this frequency the deviation from the calculated value is about 15 %.

The circuit diagram could hardly be simpler, aside from the power supply connections the output signal on pin 6 (PB1) is the only other connection necessary.The example program, written in Assembler is just 15 lines long! With a program this short comments are almost super fluous but are included for clarity. The code can be downloaded from the Elektor website [1]. 

The program only needs to initialise the timer which then runs independently of processor control to output the clock sign al . The processor can then be put into sleep mode to memory used up the remaining 99 % is free for use for other tasks if required. 

The OSCCAL register contains a calibration byte which allows some adjustment of the CPU clock. This gives a certain degree of fine tuning of the output frequency. A recommendation in the Atmel data sheet indicates that the CPU clock frequency should not be greater than 1.75 MHz otherwise timer operation cannot be guaranteed. 

The more recent ATtiny45 can be substituted for the ATtiny15. In this case the CK SEL fuses should be set to put the chip’s Timer 1 into ATtiny15- compatible mode [3]. After adjustment to the program it will now be possible to obtain a higher (or more exact) frequency from the timer, the ATtiny45’s PLL can operate up to 64 MHz. Link

Notes:
The circuit uses two 2N3819 FETs in cascode configuration. The lower FET operates in common source mode, while the upper FET, operates in common gate, realising full high frequency gain. The bottom FET is tunable allowing a peak for a particular station. Coil details follow:

This Simple Solar Flasher circuit is a single transistor fly back (Joule Thief) circuit that features a third coil. With it, flash duration and brightness is much enhanced, without resorting to large value capacitors.

Circuit Diagram: