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LED’s are always biased
A G
Inc.
Your comfort level with your response to the following two question assessment tool should indicate if the presentation that follows will increase you knowledge base on the topic outlined by the questions in this tool.
Pre-presentation Self Assessment Activity
1. Include an LED in the control scheme such that it will be ON when all of the sensors are in alarm.
Pre-presentation Self Assessment Activity
2. Size the current limiting resistor for this application to prevent the LED from burning out if the controller is a TTL system.
PSHPSL
TSL
TSH
LSHFSLFSH LSL
A useful light bulb
The Light Emitting Diode
Like any diode, it can be arranged in a circuit so that it is “forward biased” or “reverse biased”.
light when forward biased.
When the LED is forward biased it will emit light. LED’s come in various flavors. You can get one that emits red, green, yellow, or blue light when forward biased. In all of these LED’s they emit no light when they are reversed biased. Note:
The terminals of a LED are identified as an anode and a cathode.
No light when reverse biased.
Cathode terminal
Anode terminal
anode
cathode
The light emitting diode is a small semiconductor light bulb that is useful as a status indicator
A useful light bulb
Possible Light Emitting Diode circuit arrangements
5 volts
R1
A) the LED in its passive state is wired so that anode is more positive than its cathode.
LED is forward biased. LED is reverse biased.
Light is ON when button is not depressed.
LED is forward biased.
anode
cathode
A useful light bulb
Possible Light Emitting Diode circuit arrangements
5 volts
R1
A) the LED in its passive state is wired so that anode is more positive than its cathode.
LED is forward biased.
Light is ON when button is not depressed.
R1
LED is reverse biased. LED is forward biased.
the LED in its passive state wired so that the anode is not more positive than the cathode.
B)
Light is OFF when button is not depressed.
LED is reverse biased.
5 volts
anode
cathode
A useful light bulb
Possible Light Emitting Diode circuit arrangements
5 volts
R1
A) the LED in its passive state is wired so that anode is more positive than its cathode.
LED is forward biased.
Light is ON when button is not depressed.
R1
LED is reverse biased.
the LED in its passive state wired so that the anode is not more positive than the cathode.
B)
Light is OFF when button is not depressed.
5 volts
anode
cathode
The duel role of R
1
Pull up resistor - pulls the LED anode above the voltage at the LED cathode.
a)
Current limiting resistor - restricts the current flow through the LED.
b)
A useful light bulb
Possible Light Emitting Diode circuit arrangements
C) the LED in its passive state is wired so that anode is more positive than its cathode.
5 volts
R1
LED is forward biased.
Light is ON when button is not depressed.
R2
R3
V4
anode
cathodeNote;(a) V4 is also the voltage drop across the LED (b) V4 depends on the equivalent resistance at the node.
A useful light bulb
Possible Light Emitting Diode circuit arrangements
C) the LED in its passive state is wired so that anode is more positive than its cathode.
5 volts
R1
LED is forward biased.
Light is ON when button is not depressed.
R2
R3
Note;
(c) Forward biased LED’s has a very low resistance to current flow. In most situations, LED is wired in series
with single resistor.
(d)
anode
cathode
anode
cathode
RLED
V+
LED is forward biased.
anode
cathode
RLED
V+
LED is reverse biased.
V4
(a) V4 is also the voltage drop across the LED (b) V4 depends on the equivalent resistance at the node.
LED process status indicator
Circuit constraints
RLED
An LED emits light when it is forward biased and there is enough current flow to allow us to see the light.
To protect the LED, the current is always limited. As a good first approximation, the LED is presumed to contribute no resistance to current flow when it is forward biased.
Most people can see the light from an LED when the current is limited to 0.016 amperes.
TTL devices provide 5 volts at logic “1”.
5V
RLED
= 5 Volts
0.016 Amps
RLEDis approximately equal to 330 ohms.
011
011
11
LED process status indicator
If 333 (base 8) is a code that represents the signals from a set of 8 sensors that indicate that it is important to turn on an active low motor, design a circuit that will turn on an LED when that situation is present.
Typical indicator application for LED
Alarm status code from sensors
PSHPSL
TSL
TSH
LSHFSLFSH LSL
RLED
1 0 1 1 0 1 1 1 LED will only be forward biased when 333 (base 8) is represented at the inputs of the logic circuit.
LED is ON when it is forward biased.
Post Presentation Self Assessment Activity1. Include an LED in the control scheme such that it will be ON
when all of the sensors are in alarm.
PSHPSL
TSL
TSH
LSHFSLFSH LSL
RLED
LED is ON when it is forward biased.
PSHPSL
TSL
TSH
LSHFSLFSH LSL
0 0 1 1 1 1 1 0 Binary pattern when all sensors are in alarm
Post Presentation Self Assessment ActivitySize the current limiting resistor for this application to prevent the LED from burning out if the controller is a TTL system.
RLED
PSHPSL
TSL
TSH
LSHFSLFSH LSL
0 0 1 1 1 1 1 0
Logic 1 is nominally 5 volts for TTL devices.R
LED= 5 Volts
0.016 Amps
RLED
is approximately equal to 330 ohms.
RLED
5V
0.016 Amperes is a typical design constraint current limit.
2.
End of Presentation
A G
Inc.