# Optical pulse sensor or CT clamp for Solar PV monitoring **URL:** \ **Category:** Hardware\ **Created:** [4 September 2021 14:43 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535 "2021-09-04T14:43:31Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Stephen2](https://community.openenergymonitor.org/letter_avatar_proxy/v4/letter/s/2bfe46/32.png) [@Stephen2](https://community.openenergymonitor.org/u/Stephen2)\ **Post date:** [4 September 2021 14:43 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535/1 "2021-09-04T14:43:31Z")
Hi, My old geo Chorus PV monitor used an optical pulse sensor on the export meter and a CT clamps on on house feed (connected to the out of the grid meter) and additionally an optical pulse sensor on the grid meter. Not sure why the optical pulse sensor on the grid meter but… So my query is which sensor is better or do i need both optical and CT on the PV export meter. ---
**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\ **Post date:** [4 September 2021 21:06 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535/2 "2021-09-04T21:06:37Z")
You cannot have two pulse inputs on either an emonTx or an emonPi given the present design. Although the processor does have two interrupt inputs, which are what we use, one interrupt is used for the radio, leaving the possibility of only one pulse input. Therefore, to monitor two flashing LEDs, you need two devices. A current transformer on the grid connection in conjunction with a voltage transformer is able to both measure the current and by comparing the phase relationship between current and voltage, determine both the real power and the direction of power flow. An optical pulse sensor is both accurate and historical - it is as accurate as your supplier’s meter, but it can only show energy that has already passed through it. It cannot give an indication of power until some quantum of energy has been measured over some period of time and you are able to calculate the rate of change of energy over time. A current transformer is capable of calculating power from one mains cycle to the next, but as an analogue device having inherent errors, it must be calibrated carefully. When that has been done, it should be good to within about 1% of your supplier’s meter. > [@Stephen2](#): > > my query is which sensor is better Each is better than the other, depending on what you want to know from it. ---
**Author:** ![Stephen2](https://community.openenergymonitor.org/letter_avatar_proxy/v4/letter/s/2bfe46/32.png) [@Stephen2](https://community.openenergymonitor.org/u/Stephen2)\ **Post date:** [4 September 2021 22:06 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535/3 "2021-09-04T22:06:24Z")
Hi, Thanks for your reply Robert. I was not going to purchase any optical sensors. Since i have found little infor regarding them on the Community. But wanted to know if i had missed anything. As the old geo Chorus used 2 of them. ---
**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\ **Post date:** [4 September 2021 23:12 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535/4 "2021-09-04T23:12:57Z")
> [@Stephen2](#): > > I was not going to purchase any optical sensors. > Since i have found little infor regarding them on the Community. Really? Have you looked at the links on the Shop page? What “infor” do you need that isn’t on that page or linked from it? Or maybe you didn’t find these topics > [@First try with EmonPi - Pulsecount stuck at 1](https://community.openenergymonitor.org/t/first-try-with-emonpi-pulsecount-stuck-at-1/7375/16): > > Some measurements: I used a white LED reading lamp, with several layers of draughting film over the front to attenuate the light. The sensor was therefore looking at a bright but diffuse patch of light approx. 30 mm in diameter. EmonTx V3.4 Condition Output voltage Distance to light source Just detecting logic '1' 1.42 V 570 mm LED just visible 3.28 V 460 mm LED bright 3.28 V 400 mm EmonPi Condition Output voltage Distance to light source Just detecting logic '1' 1.35 V 710 … > [@Adjusting sensitivity of Optical Pulse Sensor](https://community.openenergymonitor.org/t/adjusting-sensitivity-of-optical-pulse-sensor/8399/21): > > Further investigations with Optical Pulse Sensor connected to emonTH. No external pull-up or pull down resistor connected for these measurements, but a 0.1 uF capacitor left connected between IRQ and Gnd. Vcc = 3.346 V Virq with sensor in darkness = 0.745 V Virq with sensor illuminated = 3.333 V BUT the LED on the sensor can be misleading ie in partial illumination Virq varies with light level but the sensor LED remains off. In partial illumination Virq can be as high as 3.330 V (ie high) and … > [@Stephen2](#): > > As the old geo Chorus used 2 of them. Is something missing from that sentence? ---
**Author:** ![Stephen2](https://community.openenergymonitor.org/letter_avatar_proxy/v4/letter/s/2bfe46/32.png) [@Stephen2](https://community.openenergymonitor.org/u/Stephen2)\ **Post date:** [5 September 2021 15:51 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535/5 "2021-09-05T15:51:41Z")
The Geo Chorus used two optical sensors and after reading info the emonPi/emonTx can only have 1 optical sensor. So i wasn’t sure which meter i would connect the optical sensor too. So thought i would get up and running first with 2x CT sensors and then look at adding an optical sensor. ---
**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\ **Post date:** [5 September 2021 16:20 UTC](https://community.openenergymonitor.org/t/optical-pulse-sensor-or-ct-clamp-for-solar-pv-monitoring/18535/6 "2021-09-05T16:20:20Z")
I thought something had got lost in editing, as it started with “As…”. It only makes sense to have the optical sensor on the generation meter as a permanent fitting, because the Grid meter doesn’t generate pulses when exporting. Even then, the export meter won’t generate pulses overnight when the inverter is on standby, but the power/energy consumed then is minimal. I would think you would want to use the optical sensor to calibrate both c.t’s and inputs (bear in mind that you calibrate c.t and input **as a pair** and they should thereafter be kept together) and then moved to the generation meter. But it’s your choice, bearing in mind the limitation.