Energy efficient active IR sensor’s transmitter design

Stepping it down is the way

SenseBe by Appiko, an active motion sensor whose transmitter this article explains

What should a transmitter do

In this article we’ll focus on designing the electronics for transmitter in an active Infrared (IR) motion sensor. Such a sensor detects break in an IR beam between a transmitter and a receiver, usually with an animal coming in between in wildlife camera trapping. We want an energy efficient transmitter that can output enough IR radiation to allow a separation of 10m or more between the transmitter and receiver, with configurable radiation intensity.

The primary component in the receiver is the TSSP4056 IR sensor module by Vishay Semiconductors. Its output is pulled low when an infrared light at around 940 nm modulated at 56 kHz is incident on it. To lower power consumption, pulses of IR beam with a frequency of 56 kHz is sent to the receiver and the number of pulses missed is counted to detect a body in between the transmitter and receiver. So the IR light transmitter would be according to the waveform shown below with the base frequency, base duty cycle and pulse period fixed at 56 kHz, 10% and 1 ms respectively, along with a variable down period, usually at 24 ms.

Transmitter’s IR emission pattern

Choosing the components

IR emitter

For the transmitter first we had to decide the active IR transmitting element. We could choose either LED or LASER. We decided to use a LED as it had many more advantages over a LASER emitter, namely 1.Larger field of view, thus allowing some room for adjustment in alignment of the transmitter and receiver. 2. LASER needs special certification to sell as a product. 3. LEDs are comparatively inexpensive. LASER can be more power efficient since the field of view (FoV) is quite narrow, less than a degree, so less power is radiated into the ambient surroundings. But as mentioned earlier, this can lead to difficulty in aligning the two units, especially after a few meters and this alignment can be undone by tiny movements.

With these considerations we decided to use SFH4545, a 940 nm IR LED with a FoV of 10 degrees. This FoV eases the alignment of the transmitter with the receiver during deployment while not radiating much power in surrounding directions.

Graphs that show how LED’s forward voltage (Vf) is related to forward current (If) and in turn radiation intensity

Copied above from the datasheet of this IR LED are the two graphs that help us understand its behavior. First one shows that until the forward voltage (Vf) is increased till a little over 1V, there is no forward current and post that the current shoots up rapidly reaching about 100 mA at 1.5 V of Vf. The second graph shows that the IR radiated intensity increases linearly with increase in the forward voltage. Now we can infer what the radiation intensity would be at different forward voltage.

As mentioned earlier, the pattern in which IR light is emitted from the transmitter consists of pulses of light modulated with a certain frequency. Because of this pulsed emission of light, the forward current of the LED can be higher than the maximum rated continuous current, which is 100 mA in case of SFH4545.

The graph to the left shows the maximum rated current in different scenarios of pulsing a LED. As you can see when the rated current depends both on the frequency in which the LED is driven (Inverse of T in the graph) and its duty cycle (D in the graph), where the rated current normally decreases with increase in on-time of the pulse and with decrease in duty cycle at higher frequencies (> 100 Hz). In our case the base frequency is 56 kHz (T=1/56000=0.018 ms) and we use a 10% base duty cycle (D=0.1), which is slightly higher than the specified minimum usable value of 5% by TSSP4056 sensor. Also, the pulse period is a small portion of the total beam interval, mostly in single digit percentages. This means that we could drive the LED with even 500 mA current without damaging it.

LED driver

For the LED driver the criteria that we had were that it should be energy efficient, low cost, works with dual AA batteries and able to change the LED current to control the radiated light. As seen in the Vf vs If graph earlier of the IR LED, the driving voltage is between 1.2V and 1.8V in our case. This means we need to generate this lower voltage from the supply voltage, which in our case would be dual AA batteries, either rechargeable or non-rechargeable. The two ways of doing this would be either linear or switching buck regulator. This article provide a good comparison of the two. In our case a switching regulator make more sense because of the greater efficiency and the switching noise isn’t an issue for driving a LED.

For a LED driving circuit, some of the options are:

1. A constant voltage supply higher than the forward voltage of the LED driving the LED with a resistor in series to limit the current. This is the simplest circuitry possible to drive a LED and is also forgiving to minor changes in the supply voltage. The drawback is that the power spent across the resistor is dissipated as heat and reduces the efficiency.
2. A constant current supply which makes sure that the current flowing through the LED is constant, even with varying supply voltage and LED characteristics, due to different production batches, change in temperature, aging or other degradation. This is usually done by measuring the current flowing through the LED by a by measuring the voltage across a low value series resistance called the shunt resistance. By this feedback of the LED current, the supply to the LED can be adjusted in real time to maintain constant current. We could not use this means since we could not find any off the shelf chip that can do constant current with a PWM of 56 kHz, which is relatively high frequency for driving LEDs.
3. Using a constant voltage supply directly with the LED without any series resistance is the third option. In usual conditions where the LED is more or less continuously ON, there is a chance that the LED can heat up. At a higher temperature a LED draws higher current at the same voltage, thus generating more heat, which in turn increases the current. This positive feedback can blow up LEDs, so this design isn’t useIn this design we opted for a design which would be quite risky in other circumstances, but in our case with the extremely low duty cycle, this approach is feasible due to the amount of time for the LED to cool down. The high current drawn is in bursts of a few milliseconds and can generate bursts on high intensity IR light. The benefit of this approach is that since there is no series resistance, there is no power lost there, increasing the energy efficiency. To generate this specific voltage to drive the SFH4545 LED, we use a buck regulator.

We chose the AP3405 for the buck regulator because of the simple design, relatively low cost and most importantly it can work till 2.3V input voltage, which is useful when working with a pair of rechargeable AA batteries. Also the soft-start of this regulator switches on the output in at most 300 us, which is not too long to switch on every beam interval.

Reference design for AP3405 buck regulator

The above schematic shows a reference design using the AP3405 regulator. The output voltage is set by connecting the output of a voltage divider (R1 and R2 in this case) to the feedback pin of this IC, where the voltage divider is between the output voltage and ground. As we know from a voltage divider the voltage at the FB pin is Vfb = Vout*(R2/(R1+R2)). Since the reference voltage used in the error amplifier of AP3405 is 0.6V, the output voltage will be regulated till the Vfb becomes 0.6V. So we can say that the output voltage can be calculated by Vout = 0.6*(1+R1/R2). For example to generate an output of 1.8V, we could choose R1 as 240k and R2 as 120k.

Circuitry

From the above selected components we’ve designed the circuitry shown below to drive a IR LED for the transmitter in an active motion sensor. You can find the entire schematic and design in this repo. Do note that in this design we’ve added both the transmitter and receiver in one PCB and we populate the appropriate components for either device.

There are four control signals to this circuit from the nRF52810 System on Chip (SoC), the microcontroller in this system. We’ll explain this circuitry through these four control inputs to this circuit.

Schematic of the IR transmitter circuit for an active motion sensor

• REG_EN: This active high signal controls the output of the AP3405 regulator. On setting this signal high, the time taken to set the proper output voltage is two to three hundred microseconds as mentioned by soft-start time in the data sheet. So because of this latency we cannot use this signal to generate the 56kHz PWM signal. Also this latency needs to be considered for setting the pulse period. This signal is made low in the down period for most of the power saving, where the driver consumes only a few uA as the standby current of AP3405.
• IR_LED_EN: This signal is to the gate of a n-Channel MOSFET controlling the current path of the IR LED. We send a continuous 56 kHz PWM signal with 10% duty cycle here, which the TSSP4056 receiver can detect. In the pulse period where the REG_EN is high, this PWM signal by the IR_LED_EN provides the necessary 56 kHz waveform for the transmitted IR signal for detection by the receiver. Even through this signal has this 56 kHz PWM in the down period too there won’t be any IR radiation since there is no power output from the AP3405 regulator.
• Power controls PWR1 and PWR2: While the enabling of the LED is controlled by IR_LED_EN and REG_EN, the LED current and thus the IR light intensity is controlled by PWR1 and PWR2. As explained earlier, there is a voltage divider that is used to generate the output voltage of the regulator and this constant voltage output is used to drive the LED. PWR1 and PWR2 are connected to the gate of nChannel MOSFET, which when enabled a resistor is added parallel to the bottom resistor of the voltage divider. In this case when PWR1 and PWR2 are enabled, R15 and R17 are added parallel to R14 respectively. This reduces the equivalent lower resistance in the voltage divider and thus generates higher output voltage to drive the LED.

Results

As mentioned, apart from the LED driver and the IR LED, the active motion sensor’s transmitter had a nRF2810 (SoC) along with its complementary components. The entire circuitry consumed just about 500 uA when transmitting with the following parameters:

• Pulse period of 1 ms
• Beam interval of 25 ms
• Base frequency of 56 kHz
• Base duty cycle of 10%
• LED forward current of 200 mA

With these parameters we were able to get about 7m of maximum distance between the transmitter and the receiver. With higher power setting using PWR1 and PWR2 in the LED driver this distance increased up to 12m.

Since the transmitter is more power hungry than the receiver, its battery life is the limiting one for the entire motion sensor system. We have tested that with a pair of rechargeable AA batteries this system lasted more than two months, which is in line with the measured current value.