Security is a key feature in Wireless Sensor Networks but they are prone to many kinds of attacks and one of them is Black Hole Attack. In a black hole attack all the packets are consecutively dropped which leads to the decrease in the efficiency of the network and unnecessary wastage of battery life. In this paper, we propose an exponential trust based mechanism to detect the malicious node. In the proposed method a Streak counter is deployed to store the consecutive number of packets dropped and a trust factor is maintained for each node. The trust factor drops exponentially with each consecutive packet dropped which helps in detecting the malicious node. The proposed method show a drastic decrease in the number of packets dropped before the node being detected as a malicious node.