Sigmoid function doesn't work with MNIST dataset.

Hi everybody, my net have to recognise if an image from the MNIST database represents the digit I choose.

An image is a vector of 784 values between 0 and 1. I can't perform the forward propagation because the sigmoid function returns "1" everytime.

All net dimensions are dynamic, but for debugging purposes I'm using a 2 layer net with 2 nodes on the first layer and 1 output node (I only need to see the output changing, but it doesn't!); I already tried with more layers and more nodes but I still have this problen. In the hidden layer the activation function is the sigmoid; in the output layer it is the identity function.

I tried to preprocess the input (scale from a 28x28 to a 14x14 img) but it didn't work.

Thank you in advance for your help!

[Srikanth Sedimbi]

Are you using any of the MATLAB inbuilt functions for sigmoid or did you write your own sigmoid function? Just post the code for sigmoid function and the corresponding inputs and outputs