A tale of 2 servos

I plan to control my turnouts with servos. I researched and discovered that MERG has several options for control of servos. I chose their Megapoints kit which is a kit version of a commercial offering in the UK. It can control up to 12 servos and since I have 12 turnouts on my planned module, it was a perfect fit.

The instructions indicated the Hobby King HK15178 servo was a good choice. They also indicated that a Tower Pro SG90 would be appropriate. I went with the first in the list and ordered 10 from the Hobby King website. When they arrived I hooked them up and noticed that as the Megapoints board powered up, the servos twitched quite dramatically.

I was concerned as this would cause the points to force one way, beyond the stop setting as done with the Megapoints board set up. I posted on the MERG forum and a discussion ensued. It was suggested that maybe a pull up resistor would be a method to get rid of the twitch and it seemed to work. However, an email exchange with the Megapoints company suggested I not fit this resistor to pull up the data line to 5v. I put the project aside.

Since I planned to have an additional double crossover, I needed to control 4 more turnouts. I chose the MERG Servo4 board, which as the name indicates, controls 4 servos. When I built that and hooked up the servos the twitch was there again. However, the Servo4 board comes equipped with an option to enable either no resistor, a pull up resistor or a pull down resistor. No change in the setting got rid of the twitch. I was frustrated.

Tower Pro SG90 servos are the Toyota Corolla of servos. They are everywhere because they are small, cheap and work well. There are also a TON of knock offs that are even cheaper. I decided to try a knock off and the price was right so I ordered 20 from Amazon. I hooked one up to the Servo 4 board and there was absolutely NO twitch. I went back to the Megapoints board and again, no twitch. Back to the HK servo, and the twitch was back.

I posted my experience on the MERG forum and a discussion is ongoing. All I can report is my experience. I won’t be using the HK servos for turnout control. I have 20 of the knock off SG90s and if I have a few that fail, I now have spares. If I wanted true reliability, for a few extra dollars I could buy the genuine Tower Pro SG90 servos.

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