ECE:3410 Electronic Circuits

Transcription

1 ECE:3410 Electronic Circuits IR Link Labs Textbook Blackboard A. Kruger IR Link Labs, Version 2.3 1

2 Specifications Design a simple IR remote control Press a button on a transmitter Turn on a 5 V, 50 ma, mechanical relay, located at least 6 feet away Transmitter: 9 V power supply (battery powered) Receiver: uses 912 V bench power supply. Available components 2N2222 npn BJT 2N7000 MOSFET TS555C Timer IC Standard capacitors & resistors IR diode: LTE4208 (limit average current to < 40 ma) IR photo transistor: LTR4206E Small signal diode: 1N914 Panasonic LKT1AF9V mechanical relay Data sheets and SPICE models are available on class website under Resources A. Kruger IR Link Labs, Version 2.3 2

3 Overall Design acdc conversion Use 555 timer Drive frequency = 5 khz Limit to 40 ma Use duty cycle of 70% IV conversion Block dc (ambient) light Amplify Drive load Lab 4 Lab 5 A. Kruger IR Link Labs, Version 2.3 3

4 555 Multivibrator Circuits Monostable Astable Often used to generate square waves See Design Example A. Kruger IR Link Labs, Version 2.3 4

5 Transmitter IR Diode Driver LowSide Switch LowSide Switch HighSide Switch LowSide Switch Some options to consider for the IR diode driver A. Kruger IR Link Labs, Version 2.3 5

6 Tx Detailed Schematic Astable configuration See Design Example Lowside switch with current limiting A. Kruger IR Link Labs, Version 2.3 6

7 Tx Current Limiting MOSFET is used as a switch. Drive V G >> V TN to turn it on hard When the voltage here (I D R limit ) reaches 0.7 V, then the BJT turns on and clamps V G, thus limiting further increase in I D This resistor limits I C so that the BJT is not destroyed. A. Kruger IR Link Labs, Version 2.3 7

8 Duty Cycle and Average Values v(t) Duty Cycle Fraction or % time on t Average value t Question: what is the peak value when 0.55 and the average is 30 ma? Answer: ma A. Kruger IR Link Labs, Version 2.3 8

9 Tx Detailed Schematic Small but important detail Without the decoupling capacitor, large voltage spikes may appear on the supply rail. This may propagate into the 555 timer and reset it. A. Kruger IR Link Labs, Version 2.3 9

10 Completed Transmitter A. Kruger IR Link Labs, Version

11 Completed Transmitter 555 Timer IC Pushbutton Switch and On/Off indicator LED A. Kruger 2N2222 BJT IR LEDs 2N7000 MOSFET IR Link Labs, Version

12 Completed Transmitter GND A. Kruger IR Link Labs, Version

13 555 Timer Internals Vcc (8) Reset (4) Thresh (6) Ctrl (5) R Q Out (3) Trig (2) S Q Discharge (7) 1 (GND) A. Kruger IR Link Labs, Version

14 555 Timer Internals Comparators Vcc (8) Reset (4) Thresh (6) Ctrl (5) R Q Out (3) Trig (2) S Q Discharge (7) 1 (GND) so that this chain generates 3 and 2 3 A. Kruger IR Link Labs, Version

15 555 Timer Astable RESET R Q 3 2 S Q 7 1 A. Kruger IR Link Labs, Version

16 555 Timer Astable RESET R Q 3 2 S Q 7 1 A. Kruger IR Link Labs, Version

17 555 Timer Astable RESET R Q 3 2 S Q 7 1 A. Kruger IR Link Labs, Version

18 555 Timer Astable RESET R Q 3 t 2 S Q 7 t 1 charges through and until towards A. Kruger IR Link Labs, Version

19 555 Timer Astable RESET R Q 3 t 2 S Q 7 t 1 When, the upper comparator trips, resets the FF, goes low, and goes high. This turns the FET on and starts to discharge through towards GND.. A. Kruger IR Link Labs, Version

20 555 Timer Astable RESET R Q 3 t 2 S Q 7 t 1 When falls to, the lower comparator trips. This sets the FF, and goes high and goes low. A. Kruger IR Link Labs, Version

21 555 Timer Astable RESET R Q 3 t 2 S Q 7 t 1 Now the FET is off, and starts to charge again through and. A. Kruger IR Link Labs, Version

22 555 Timer Astable RESET R Q 3 2 S Q 7 1 Be sure to decouple the power supply and pin 5. In most cases, a 0.1 F capacitor will work. A. Kruger IR Link Labs, Version

23 Receiver Block Diagram Detector Buffer Gain Block Detector Output Driver A mv mv V V V I V conversion Biasing Filtering High R o Filtering High R i Low R o Main voltage amplifier Filtering AC/DC convert Detect level Filtering Power Relay Lab 5 A. Kruger IR Link Labs, Version

24 Average (dc) value depends on ambient light in room. Detector Signal Detector A mv 120 Hz from incandescent lamp 5 khz from transmitter riding on much larger 120 Hz from incandescent lamp A. Kruger IR Link Labs, Version

25 Frequency Domain View 120 Hz from incandescent lamp 5 khz from transmitter riding on much larger 120 Hz from incandescent lamp Amplitude (V) Ambient Lamp Computer Screen, TV, ~ 0Hz 120 Hz 5 khz Frequency (Hz) IR Tx A. Kruger IR Link Labs, Version

26 Frequency Domain View 120 Hz from incandescent lamp 5 khz from transmitter riding on much larger 120 Hz from incandescent lamp Amplitude (V) Noise Signal ~ 0Hz 120 Hz 5 khz Frequency (Hz) We will improve the SignaltoNoise Ratio (SNR) using filtering A. Kruger IR Link Labs, Version

27 Frequency Domain View Amplitude (V) ~ 0Hz 5 khz Frequency (Hz) Filter Response Amplitude (V) ~ 0Hz 5 khz Frequency (Hz) A. Kruger IR Link Labs, Version

28 Detector Buffer Gain Block Detector Output Driver A mv mv V V V I V conversion Biasing Filtering High R o Filtering High R i Low R o Main voltage amplifier Filtering AC/DC convert Detect level Filtering Power Relay A. Kruger IR Link Labs, Version

29 Detector Buffer We will use a phototransistor rather than a photodiode. Phototransistors are times more sensitive than photodiodes, but are nonlinear and slower. However, in this application these factors are less important. converts the photo current (few A to small (~ mv) voltage. The larger, the bigger the photovoltage, so for sensitivity one wants it to be large. However, increasing also increases the dc photo current and dc voltage across it. If it is made too large, the dc voltage across it will approach the power supply. Start with a value = 10K, and once the complete receiver works, come back and adjust its value. The coupling capacitor blocks dc. It forms a time constant with and the input resistance of the buffer stage. One can use this to filter out 120 Hz noise. A. Kruger IR Link Labs, Version

30 Detector Buffer Use a 2N7000 MOSFET configured as a source follower as a buffer. The buffer will have a voltage gain < 1, but can have a very high input resistance. This is important because we don t want to load. Since ~ 10K, design the source follower so that its input resistance is much larger. We want the source follower s output impedance small so that the source follower can drive the subsequent gain block. The gain block s input resistance is on the order of 2.5K. The follower s output resistance should be much smaller than this. Note that the follower s output resistance depends on and. Additionally, we want the dc voltage across to be roughly midway between the power supply rails. This allows for the output voltage to swing symmetrically before being clipped. Start the design by picking a reasonable. A. Kruger IR Link Labs, Version

31 Buffer Gain Block Use a 2N2222 BJT for the gain block. We want the dc voltage across to be roughly midway between the power supply rails. This allows for the output voltage to swing symmetrically before being clipped. Start the design by picking a reasonable. Choose so that it is a short at the operating frequency (5 khz). The coupling capacitor sees the output resistance of the follower and the input resistance of the gain block and forms a time constant. One can design for to provide additional filtering to pass the 5 khz signal and suppress the 120 Hz noise. A. Kruger IR Link Labs, Version

32 Detector A. Kruger IR Link Labs, Version

33 SPST NO Relay Single Pole, Single Throw, Normally Open relay. The actual relay is a DPDT but you can use as a SPST Diode suppresses voltage transient when relay turns off TE Connectivity Potter & Brumfield Relay, part number Digikey part number PB383ND Add a circuit here to ensure that no more than 50 ma flow through the MOSFET. A. Kruger IR Link Labs, Version