Adjustable triangle/sawtooth wave generator utilizing 555 timer

The circuit works on account of the truth that voltage will increase or decreases linearly when a continuing present flows into or out of a capacitor. If we handle to cost and discharge the capacitor with the identical fixed present, then the voltage of the capacitor will turn into a triangular wave; that’s the crux of this circuit (Determine 1).

Determine 1 Circuit schematic of triangle/sawtooth wave generator utilizing 555 timer.

Assuming that the circuit has reached steady-state, then the working precept is as follows: When the voltage throughout capacitor is 1/3V_cc, the 555’s output shall be excessive, making D1 and D4 reverse biased however D2 and D3 ahead biased. Due to this fact the capacitor will cost by D2 with a continuing present I₁ (its voltage will improve linearly) till its voltage reaches 2/3V_cc. When that occurs, the 555’s output shall be low, making D3 and D2 reverse biased however D4 and D1 ahead biased, thus discharging the capacitor by D4 with a continuing present I₂. If I₁ is the same as I₂, then it takes the identical period of time to go from 1/3V_cc to 2/3V_cc and vice versa. Nevertheless, if I₁ is totally different from I₂, we will generate totally different waveforms equivalent to constructive and unfavorable sawtooth waves.

The expression for the rise time (T_RISE) and fall time (T_FALL) is the next:

                                                                 [1]

                                                                [2]

If we make I₁ and I₂ depending on V_cc, we will remove the dependency on V_cc, this makes the above expressions solely depend upon fastened parameters (extra on that later).

For simulation functions let’s assume that C = 100nF, I₁ = I₂ = 2mA and V_cc = 9V, due to this fact T_RISE = T_FALL = 150µs_. As proven in Determine 2, the circuit works correctly.

Determine 2 Simulation of triangle/sawtooth wave generator circuit proven in Determine 1.

Let’s now transfer to the actual circuit and take a look at some actual waveforms. The essential precept is similar however I₁ and I₂ are changed by present sources created with one LM358 and two transistors (Q1 and Q2) (Determine 3).

Determine 3 Schematic of triangle/sawtooth wave generator with an LM358 and two transistors used as present sources.

Now we’ve got to consider that Q1 and Q2 by no means enter saturation. The utmost collector voltage of Q1 shall be round 2/3V_cc + 0.7V since V_cc = 9V. Due to this fact, we’ve got to design the circuit in order that the emitter voltage is increased than or equal to 2/3V_CC + 1V to keep away from saturation. This identical precept is utilized to Q2. (Nevertheless, now it’s the minimal voltage on Q2’s collector which is round 1/3V_cc – 0.7V, so we’ve got to design an emitter voltage lower than or equal to 1/3V_cc – 1V.) Because of the unfavorable suggestions of the op-amp, it’s simple to do with a voltage divider (eradicating the dependency on V_cc in equations 1 and a couple of) for the reason that non-inverting and the inverting enter are on the identical voltage.

On this explicit case, I₁ = I₂ = 2mA, so we should always have T_RISE = T_FALL = 150µs. Let’s check out the triangular waveform (Determine 4):

Determine 4 Scope seize of triangular waveforms from the circuit proven in Determine 3.

So the circuit works correctly, but when I add it in collection with R1 and R4—two 10kΩ potentiometers—we will change the full resistances on the emitters and due to this fact change the present I₁ and I₂. If, for instance, we transfer the potentiometer of Q2 to roughly 0Ω and the potentiometer of Q1 to 10kΩ, then I_{2 =} 2/1kΩ = 2mA and I_{1 =} 2/11kΩ = 181µA, due to this fact it’ll take longer (11 occasions longer, round 1.65ms) to cost the capacitor so we’ve got generated a constructive sawtooth wave (Determine 5):

Determine 5 Optimistic sawtooth wave generated from circuit proven in Determine 3.

Enjoying with the potentiometers permits you to generate an arbitrary waveform (inside limits). After all, decreasing R4 or R1 will trigger the I₁ and I₂ currents to extend and cut back the minimal falling or rising time.

To see the circuit in motion you’ll be able to verify the next video:

Associated Content material