Good day everyone, I'm currently planning a project on a low-cost WiFi-enabled RGB LED controller and could use some help in looking over my parts list and general concept. What I'm wanting to do is a build a relatively low-cost, WiFi-enabled RGB LED (strip) controller. In the end it should be controlled via an Android App which is already being developed in my free time. For the WiFi module I want to use the infamous ESP8266 board, namely an ESP-07 (http://www.banggood.com/ESP8266-ESP-07-Remote-Serial-Port-WIFI-Transceiver-Wireless-Module-p-968191.html). The plan is to flash it with nodemcu (http://nodemcu.com/index_en.html), write a simple TCP server implementation which allows to manipulate 3 GPIO pins' duty cycle over the network. I have no problems related to the software or the programming of either the MCU, the App or anything else. What I'm unsure about are the electronics involved here. I've come to understand that I want to use transistors or MOSFETs to drive the LED Strips 12v using the 3.3v logic of the ESP8266. From what I know, I should choose a FET that can be switched fast (requirement for PWM) and has a maximum gate threshold voltage below 3.3 Volts. Based on these requirements and the availability at my supplier I've chosen the IRFZ44N (http://www.irf.com/product-info/datasheets/data/irfz44n.pdf) and I want to wire it directly to the three GPIO outputs of the Microcontroller. Is there any reason for this being a bad idea? Should I maybe add a diode? I don't think so since LEDs are resistive load and not inductive (so there shouldn't be voltage spikes or any of the likes, but I'm a layman when it comes to electronics, far more involved in programming..) Please tell me if there are any pitfalls to this plan and thank you for taking the time to read through my plan. - rjhllr
The large MOSFET transistors have a large gate capacity, usually 1nF or so. The direct connection to the microcontroller causes high current flowing when the signal changes from LOW to HIGH and vice versa. You should check the maximum current that the microcontroller drives. Most microcontrollers limit the current already. For example ATmega controllers drive approximately 40mA on each pin but the sum of all currents must be below 200mA. So for an Atmega, it would be Ok to do that with 3 pins (3x40mA + a little other current is far below 200mA). The Power supply must be able to deliver that current without becoming unstable. Your peak current on the 3,3V line will be much higher thatn the average current. The peak could be 100x higher than the average current. You can reduce the peak current by putting a resistor in series to the MOSFET's gate. But then then charge and discharge times increase and therefore you loose more energy in the MOSFET's which makes them warmer or hot. You may use MOSFET driver chips and P-Channel mosfets. Thus you can drive the MOSFETS with the unstable high-voltage of the LED stripes while keeping the load of the 3 logic signals from the microcontroller at a minimum. Last you should check the temperature in a long-time running test under realistic conditions.
Thank you for the thorough and clear explanation Stefan, What I was (and still am) worried about is the MOSFETs gate voltage threshold, according to the Typical Output characteristics graph I fear that it wouldn't even switch at 3v3 logic level at its gate. There is no line for 3v3, it starts at 4.5V. What I thought about to make everything more stable (and make sure it switches) is to give the MOSFET 12v from the same PSU as the LED strips and switch it with another transistor, but I cannot for the love of god figure out which transistor type and characteristics to choose to switch the MOSFET more reliably.
Hi, I've made a project similar to yours and present it on this website: http://www.esp8266color.com/ Don't hesitate to take or improve the design and code there. The PCB and code are on github and everything is available open source. The PCB can also directly be ordered from DirtyPCB. I'll try to improve the site and complete it with more information, also adding some alternative code for whom doesn't want to use souliss framework.
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