- Published in issue 3/1992 on page 58
555-based ramp generator
Whilst developing a simple scanning receiver, a need occurred for a ramp generator to provide a varying voltage to apply to the varicap diodes fitted to the variable frequency oscillator (VCO). Referring to various literature suggested that a unijunction transistor (UJT) could be used to produce a reasonable 'sawtooth' waveform that would da the job. The circuit found provided a ramp upwards from about 1/2 Vcc only, and in this case did not provide a sufficient range of voltage (and, therefore, scan) on the receiver. Since all the UJT circuit was doing was allowing a capacitor to charge/ discharge (and was, therefore, not linear anyway), thoughts tumed to other methods, hopefully cheaper and more flexible.