![]() C13 is used as a DC block on the output and R15 assures that the op amp's output will always see a resistive load to assure stability. Since U2b has some gain, the values of C10 and R11 were chosen to attenuate the input to U2a enough so that even at a relatively low battery voltage (7.0 volts) it avoids clipping when hitting the power supply rails, providing about 4.5 volts RMS. U2a is used as a unity-gain follower to buffer the rather high impedance output from the initial R/C PWM filtering (R9-11, C8-C10) and it feeds U2b which is configured as a 2 pole 1 kHz bandpass filter to further clean up the 1 kHz sine wave being generated. 7 volts for this application) and it is capable of driving 600 ohm resistive loads. U2, a TS462, is a "rail-to-rail" op amp, this type being chosen to maximize the output signal level available from the limited supply voltage, particularly when the 9 volt battery is nearing the end of its useful life (approx. To the collector of Q2! (I'll get around to fixing it some day.) NOTE: There is an error in the schematic: The anode of D1 should have been shown connecting Temperature-stable (plastic) units rather than ceramic. The only critical components are C10-C12 which should be R11 is used to establish an operating point for U2 and is derived from a mid-supply voltage source produced by R5 and R6 and filtered by C2 and ultimately used to bias both halves of U2 to a mid-supply voltage. ![]() ![]() The PWM signal is output from pin 5 using the PIC's hardware, the DC is blocked with C8 and then low-pass filtered in two stages, first by R9/C9 and again by R10/C10 to both (mostly) remove the PWM's 20 kHz energy and also to somewhat attenuate higher-level harmonics of the 1 kHz tone itself. Diodesĭ1 and D2 provide isolation for the pushbutton so that the PIC canĭetect if the user is pressing the button independently of what Q2 is (>1 second) pin 7 is set high which activates Q2 which, in turn keeps Q1 turned on. Once the software in the PIC (U1) hasĭetermined that the button has been held down for a long enough time The main power switch is Q1, a PNP transistor in series with theīatt+ lead and when SW, a momentary pushbutton switch, is pressed, Q1'sīase is pulled low through R2 causing it to conduct and provide power to Throwing a piece of paper on the workbench, I sketched out a circuit that quickly evolved into that depicted below in Figure 2. I wanted this device to be quite small and battery-powered as it would be kept with the service monitor at all times. Hardware PWM generator and an A/D converter. it is necessary to generate both the subaudible tone and a standard 1 kHz tone - something impossible when you have exactly one tone generator, as is the case of my '4031 without this optional extra.įortunately, this "extra" tone generator need not do too much: Simply generating a low distortion, 1 kHz sine wave will suffice! After all, many service monitors have, as their "second" tone generator, one that generates only a 1 kHz tone!ĭecided to throw a PIC at the problem, specifically the PIC12F683, anĨ-pin processor that has quite a few useful features, most notably a The problem is this: Many amateur repeaters require subaudible tones for access, so when testing the receiver for things like SINAD, sensitivity, etc. At 4.5 volts, a smaller resistor can be used.Over the years I have collected a number of service monitors (see the postings about the IFR-1000 and Cushman CE-50A) but the one that I use most often is a rather strange bird, a Wavetek/Schlumberger 4031. The 51 ohm resistor limits the current to less than 200 mA to prevent overloading the timer output at 9 volts. The series capacitor (100 uF) increases the output by supplying an AC current to the speaker and driving it in both directions rather than just a pulsating DC current which would be the case without the capacitor. In the circuit on the right, the speaker is directly driven from the 555 timer output. Lower volume levels can be obtained by adding a small resistor in series with the speaker (10-100 ohms). Lower frequencies can be obtained by increasing the 6.2K value, higher frequencies will probably require a smaller capacitor as R1 cannot be reduced much below 1K. Frequency is about 1.44/(R1 + 2*R2)C where R1 (1K) is much smaller than R2 (6.2K) to produce a near squarewave. A small capacitor is used at the transistor base to slow the switching times which reduces the inductive voltage produced by the speaker. In the circuit on the left, the speaker is isolated from the oscillator by the NPN medium power transistor which also provides more current than can be obtained directly from the 555 (limit = 200 mA). This is a basic 555 squarewave oscillator used to produce a 1 Khz tone from an 8 ohm speaker.
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