2420 Light Sensor Amplifier

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The 2420 Light Sensor Amplifier converts the current (μA) signal from the light sensor into a voltage that can be measured by most dataloggers and system controllers. Two amplifier models are available:

Precautions: The 2420 Amplifier is weather resistant with the lid properly attached, but if it is to be left outdoors and unattended for long periods of time, it should be installed in an enclosure or sheltered location.

2420 Amplifier Gain Settings

The 2420 Amplifier provides 15 discrete gain settings to accommodate a variety of full-scale light intensities, full-scale voltage ranges, and sensor types. This section shows how to determine the correct gain settings and voltage multiplier. Gather the following information:

  • Calibration constant for your light sensor (C)
  • Maximum full-scale light to be measured (Imax)
  • Full scale input voltage of the datalogger or 5 V-- whichever is lower (Vmax)

Follow these steps:

  1. Calculate the ideal amplifier gain (Gideal).
  3. Example: A sensor with the following parameters:
    • Calibration constant:
      C       = 6.5 μA per 1000 μmol m–2 s–1
    • Full-scale light intensity:
      Imax = 2000 μmol m–2 s–1
    • Data logger full-scale channel voltage or 5 V (which ever is lower): Vmax = 5.0 V
  6. Select the gain setting (G) from the Gain Settings Table that is less than or equal to the ideal gain from step 1.
  7. Example: The ideal gain computed in step 1 is Gideal = 0.3846 V μA–1. On the table, the closest actual gain that is less than or equal to this value is G = 0.375 V μA–1.
  8. Use a number 2 Phillips screwdriver to remove the amplifier lid. Alternate the four screws, pulling the lid up as you go so that the screws do not bind with the lid.
  9. Using a small screwdriver, set the switches in the center of the circuit board based on the amplifier gain determined in step 2.
  10. Example: The gain determined in step 2 (G = 0.375 V μA–1) requires all switches to be in the off position:
G=0.375: all switches off
2420 Gain Settings Table
DIP Switch Gain (V/μA) DIP Switch Gain (V/μA)

G = 0.375 (all switches off)

 G = 0.175
G = 0.350 G = 0.150
G = 0.325 G = 0.125
G = 0.300 G = 0.100
G = 0.275 G = 0.075
G = 0.250 G = 0.050
G = 0.225 G = 0.025
G = 0.200

Do Not Use (all switches on)
  1. Re-install the lid. Torque the screws to 0.45 Nm (64 oz-in.) if using a torque screwdriver.
  2. Calculate the voltage multiplier (M). The voltage multiplier is used to convert the voltage measured by the datalogger into a light intensity. For Quantum sensors, the units for M are μmol m–2 s–1 V–1.
  4. Example: Calculate M using G = 0.375 V μA–1 from step 2 and C = 6.5 μA per 1000 μmol m–2 s–1 from step 1:

Connecting to a Datalogger

NOTE: The 2420 Amplifier requires a power supply, usually from the data logger (white wire, +3.8 to +28 VDC). The logger should wait a minimum of 0.12 seconds (120 ms) after providing power before reading the output voltage from the amplifier.

Figure 1. Amplifier Output Terminal Block Wiring

The 2420-BNC and 2420-BL Amplifier output wires (included) can be connected to a data logger in both differential and single-ended configurations, as shown in the following diagrams. The differential configuration can give better noise rejection and lower offset voltages.

NOTE: Avoid extending the output wire length. The amplifier and data logger should be kept close together to avoid excess voltage drop and the introduction of noise.

Data Logger Wiring, Differential

Data Logger Wiring, Single-ended

Important Note! In the single-ended configuration, use the following steps to check for ground loops. This procedure only applies when the 2420 is in the single-ended configuration.

  1. Disconnect the light sensor from the amplifier.
  2. Using a multiplier of 1 and an offset of 0 in the data logger program, monitor the "dark offset" mV measurement from the amplifier.
    • If the dark offset is >1 mV, try disconnecting either the brown or black lead (but not both) to minimize the offset.
    • If the offset is minimized by removing either the brown or black wire, then move this wire off to the side and cover it with a piece of electrical tape.
  3. Reconnect the light sensor to the amplifier and reset the multiplier and offset in the data logger program.

Connecting a Light Sensor

Important Note: The 2420 Amplifier requires a current (μA) signal from the light sensor. It will not work with a millivolt adapter or with a light sensor that produces a voltage output.

LI-COR quantum, pyranometer, and photometric sensors are compatible with the 2420 Amplifier. These sensors come with bare leads or a BNC connector.

2420-BNC Light Sensor Amplifier: connects to a BNC type light sensor. Attach the BNC connector to the BNC input port on the Amplifier.

2420-BL Light Sensor Amplifier: connects to a bare leads type sensor with these steps:

  1. Use a number 2 Phillips screwdriver to remove the amplifier lid. Alternate the four screws, moving the lid up with the screws so that the screws do not bind with the lid.
  2. Loosen (but do not remove) the black plastic nut on the input port.
  3. Feed the sensor cable through the nut and input port far enough that the black shielded portion extends inside the amplifier, then hand tighten the nut.
  4. Press down the connector’s spring release and insert the sensor wires into the terminal block as shown below.
  6. Re-install the lid. Torque the screws to 0.45 Nm (64 oz-in.) if using a torque screwdriver.

Equation Summary

Output Voltage

The 2420 Light Sensor Amplifier output voltage is calculated based on the equation:

Variable Units Description
Vout V Amplifier output voltage
G V μA-1 Amplifier gain setting
i μA Light sensor photocurrent signal

Ideal Gain

The ideal gain (Gideal ) is the gain needed by the 2420 Amplifier to scale the full-scale light sensor output to the full-scale input voltage of the data logger. The 2420 Amplifier uses 15 discrete gain settings, so the ideal gain must be rounded down to the nearest supported gain of the 2420 Amplifier. Ideal gain is computed with:

Variable Units Description
Gideal V μA-1 Ideal amplifier gain
Vmax V Datalogger full-scale input voltage
Imax μmol m-2 s-1 Quantum sensor full-scale light
W m-2 Pyranometer full-scale light
klux Photometric sensor full-scale light
C μA per 1000 μmol m-2 s-1 Quantum sensor calibration coefficient
μA per 1000 W m-2 Pyranometer calibration coefficient
μA per 100 klux Photometric sensor calibration coefficient

Voltage Multiplier

The voltage multiplier M converts the voltage measured by the datalogger into a light intensity. The multiplier is found by:

Variable Units Description
M μmol m-2 s-1 V-1 Quantum sensor voltage multiplier
W m-2 V-1 Pyranometer voltage multiplier
klux V-1 Photometric sensor voltage multiplier
G V μA-1 Amplifier gain setting
C μA per 1000 μmol m-2 s-1 Quantum sensor calibration coefficient
μA per 1000 W m-2 Pyranometer calibration coefficient
μA per 100 klux Photometric sensor calibration coefficient

Note: The amplifier is not compatible with the LI-210R Photometric Sensor in indoor applications due to limitations in the resolution of most dataloggers. Contact us if you have questions about the suitability of the 2420 or the LI-210R for your application.