Physics 116B TLC555 Timer Circuit

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1 Physics 116B TLC555 Timer Circuit Physics116B, 1/17/07 D. Pellett 1

2 TLC555 Timer Circuit Variation on widely-used 555 timer using MOSFETs rather than BJTs Can be used to make (among other things): Schmitt trigger astable multivibrator (relaxation oscillator) monostable multivibrator ( one-shot circuit) a simple timer based on an RC network The basic circuit operation can be described without delving fully into its internal logic 2

3 TLC555 Functional Description Functional Block Diagram VDD 2 < > >2 n-channel enhancement-mode MOSFET > < 2 We will set VDD = 5 V so logic levels will be 0 V () and 5 V () Connect Reset to ground, leave Control unconnected Circuit performance described by shaded area in Threshold and Trigger are analog inputs in range 0 to 5 V Output is a logic level: 0 V or 5 V Discharge is connected to an n-channel MOSFET drain whose source is grounded. Discharge is grounded only when Output is. Otherwise, it is an open circuit ( floating ). 3

4 TLC555 as Schmitt Trigger Functional Block Diagram VDD VIN 2 < > >2 n-channel enhancement-mode MOSFET > < 2 Connect Threshold and Trigger together to form a single input VIN. We get an inverting Schmitt trigger with VD = 2 and VU = You can see this from the table. Start with VIN = 0 V. Since Trigger <, Output is ( = 5 V) Now increase VIN. Output does not go low until VIN crosses VD = 2 Output remains as previously () for < VIN < 2 It remains low as VIN increases to 5 V. Now decrease VIN toward 0. does not go high until VIN falls below VU =. 5 V 0 0 VU V D Schmitt trigger response curve 5 V VIN 4

5 TLC555 as Relaxation Oscillator VDD R VIN 2 < C n-channel enhancement-mode MOSFET > >2 > < 2 Again, connect Threshold and Trigger together to form a single input VIN and get an inverting Schmitt trigger with VD = 2 and VU =. Connect a capacitor C between VIN and ground and a resistor R between VIN and. Just as we showed in class, we have converted an inverting Schmitt trigger into a relaxation oscillator. If the output is, C charges toward 5 V until VIN reaches VD, at which point the output goes. Then C discharges toward 0 V until VIN reaches VU and the output goes. The process continues. 5 V 0 0 VU V D Schmitt trigger response curve 5 V VIN 5

6 Relaxation Oscillator Circuit TLC555 Pinout Note Reset is connected to VDD ( active low input) The Control input is bypassed to ground with a.01 μf capacitor to prevent pickup of interfering signals (glitches) It is also a good idea to put a.01 μf capacitor between VDD and ground at the chip to filter out glitches on the power supply connection 6

7 TLC555 as One-Shot Pulse Generator R VDD VDD VC VIN = 5V C 2 < > >2 > < 2 We do some rewiring of the relaxation oscillator to get a One-Shot (monostable multivibrator). Trigger has been disconnected from Threshold and R connects to VDD (5 V) instead of Output. Threshold is connected to Discharge, which shorts the capacitor and forces VC = 0 when Output is low (see table). Now Trigger serves as the input VIN, normally set (5 V). If VIN remains high and the output is high, C will charge through R toward VDD (5 V) until Threshold reaches 2. Then Output goes low, Discharge and Threshold are grounded, and VC = 0 V. This is the stable state (VIN high, low). 7

8 TLC555 as One-Shot Pulse Generator R VDD VDD VC VIN = 5V C 2 < > >2 > < 2 Start in the stable state: VIN high, low, Discharge and Threshold grounded, and VC = 0 V. A short pulse on the input causing VIN to fall below will set Output high, ungrounding Discharge and allowing C to charge through R toward 5 V. The output remains high while C is charging. When VC (Threshold input) reaches 2, Output goes low again, shorting C to ground, terminating the output pulse and returning the one-shot its the stable state. The result is an output pulse whose width T is related to the RC time constant. 8

9 One-Shot Circuit for Lab TLC555 Pinout 10 μs positive input pulse (TTL level) Pulse inverter and stretcher Suppose In is held at 0 V for a long time. Since the BJT is cut off, the 555 Trigger input is pulled high by R (charging C so Trigger 5 V). Now the one-shot is in its stable state with Output low. A short positive input pulse forces the BJT into saturation (like a closed switch) discharging C. This pulls the Trigger input low momentarily and starts the TLC555 output pulse. The R C time constant is long enough to assure that the TLC555 is triggered (input circuit acts as pulse stretcher as well as an inverter). The width of the one-shot output pulse T equals the time required for the voltage VC across the timing capacitor C to go from 0 V to 2/3 VDD. VC One-shot circuit T Output pulse of width T 9

Multivibrators. Department of Electrical & Electronics Engineering, Amrita School of Engineering

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