1. New LED driver Preliminary tests
G. Mitev, E. Noah
1st September 2016

2. Master timing control unit
New LED driver
Oscilloscope
Optical Pulse Control unit
Timing pulse channel 1, 2 & 3
Optical pulse feedback
LED tube & fiber bundle coupling
3- channel amplitude control unit
Optical fiber bundle
Amplitude control bus I2C
Onboard USB & WiFi to RS-232 bridges
Master timing control unit

3. LED driver modules block diagram
To the remote addressable LED probes
Local control and display
dsPIC33EP64GS504
Wi-Fi module
ESP8266
USB - UART
FT232RL
Real Clock
DS1307
Non-volatile memory FM24VN10
SCI1
SCI2
I2C 2
Control measurement circuit
I2C 1
PULSE1 & PULSE2
control
Chanel 1
Chanel 2
Chanel 3
MS0
MS1
ADC T1
LMH6559 buffer
Power control
MUX
I2C buffer
Round board - interface and signal distribution
PULSE
From the Main unit
I2C
Addr. Sel.
I2C - PIO
Level translator
DAC1
DAC2
DAC3
LED Driver 1
LED Driver 2
LED Driver 3
Current mirror
Current mirror
Current mirror
LED1
LED2
LED1
BLUE
GREEN
RED
Rectangular boards – LED driver
I2C
Pulse
I2C
Pulse
I2C
Pulse
System controller block diagram
LED tube unit block diagram

4. Test goals Preliminary tests
explore light pulse characteristics of NEW LED driver
compare with old LED driver

5. NEW LED “Torch” setup
“LED driver controller” controlled manually from push buttons installed on the module (i.e. not connected to a PC). sends “timing pulse” to LED driver.
One LED interface board “round board”.
One LED driver board “rectangular board” with one high brightness LED installed on it.

6. PIN diode light output measurements
Some signal is observed on the PIN diode (green trace 4), but it’s hard to observe any signal parameters due to the noise.
It can be seen that the light emission starts and stops immediately with the current flowing through the LED.
It was determined that the aluminium foil, used to wrap the LED & PIN setup (inside the red heat-shrink tube), is acting as a wave guide and is channeling the electrical noise from the LED to the PIN diode.
Trace 1 – 30ns timing pulse
Trace 2 – LED current
Trace 3 – LED voltage
Trace 4 – PIN diode output

7. PIN diode light output measurements
The aluminium foil was removed and the coupling between the PIN and the LED was much worse. The observed signal is much cleaner.
The PIN signal clearly has 2 regions with different slopes, raising the question for their origins.
Trace 1 – 250ns timing pulse
Trace 2 – LED voltage
Trace 3 – PIN diode output

8. Noise issues with first tests
We connected a Hamamatsu S5973 photodiode to one ADVANSID amplifier board (ASD-EP-EB-N), and brought the diode in contact with the LED.
Lots of noise was observed on the diode signal.
This noise is actually electrical noise originating from the LED driver “rectangular” board.
Changed setup: LED to one connector of rectangular scintillator slab with WLS, MPPC S C to other side of connector.

9. MPPC S C: Dark noise
One photo-electron corresponds to ~50 mV

10. Timing pulse width = 14.8 ns
This is the minimum timing pulse width that leads to a measurable output on the MPPC
Amplitude set to maximum:
both in software,
... and with the potentiometer on the hardware rectangular LED carrier board

11. Timing pulse width = 15.8 ns
Measured output increases to between 10 and 50 p.e.

12. Timing pulse width = 16.9 ns

13. Timing pulse width = 17.8 ns

14. Old LED driver: Pulse train mode
Settings required for measureable light output at MPPC
Pulse train mode (max. freq. = 10 Hz):
Amplitude setting: 6
Pulse width setting: 200ns
Pulse period setting: 127 ms
Note: a fiber bundle was used for these tests

15. Old LED driver: hardware mode
Settings required for measureable light output at MPPC
Hardware mode (min freq. = 20 kHz):
Amplitude setting: 11
Pulse width setting: 400ns
Pulse period setting: 127 ms