preventing the raspberry pi wlan stick goes to sleep

config_wlan.sh

#steps to prepare heartbeat script
#assuming that you use the Edimax EW-7811Un

sudo nano /etc/modprobe.d/8192cu.conf

#add the following lines

# Disable power management
options 8192cu rtw_power_mgnt=0 rtw_enusbss=0

# make executable chmod +x /path/to/heartbeat_script

#finally make a cronjob e.g. all 30 minutes
crontab -e

30 * * * * sudo sh ~/cronjobs/wlan_heartbeat.sh >/dev/null 2>&1

wlan_heartbeat.sh

#!/bin/bash
# check if a wlan0 if exists
if echo `ifplugstatus -a` | grep -E 'wlan0' $1; then
    #check if there is IP Address
  if echo `ifconfig $1` | grep -oE "\b([0-9]{1,3}\.){3}[0-9]{1,3}\b"; then
    exit 0
  fi
  modprobe -r 8192cu
fi
modprobe 8192cu
exit 0